13 Nov 2026
ETF Glossary: Plain-English Definitions of ETF Terms
This glossary defines 157 terms that investors meet when researching exchange-traded funds, from how ETF shares are created and priced to how option income, buffers and thematic investing work. Each entry starts with a direct answer, and most link to a longer article that explains the idea in context. Terms are listed from A to Z.
Jump to: A | B | C | D | E | F | G | H | I | L | M | N | O | P | Q | R | S | T | U | V | W | #
A
Accelerated buffer
An accelerated buffer is a buffer design that gives the investor a multiple of the index’s gains up to a lower cap, in exchange for a smaller upside range than a standard buffer. The buffer absorbs losses within its stated range, and the cap sets the maximum return over the outcome period. Read more.
Active ETF
An active ETF is an exchange-traded fund whose managers choose holdings or strategy rules, rather than copying an index. Active ETFs can aim to differ from a benchmark in risk, income or return pattern. They usually carry higher expense ratios than passive funds and their results depend on the manager’s decisions. Read more.
Actuator
An actuator is a component that converts electrical, hydraulic or pneumatic energy into motion, such as turning a robot’s joint or moving a limb. Humanoid robots use dozens of actuators, so they are among the largest cost items in a robot’s bill of materials, along with sensors, batteries and computing hardware. Read more.
Advanced packaging
Advanced packaging is the set of manufacturing processes that join several chips into one module, such as placing an accelerator and stacked memory side by side on a shared base. It lets chips exchange data faster than separate packages can, and capacity for it has been a constraint in AI chip supply. Read more.
AI data center
An AI data center is a facility built to train and run artificial intelligence models, using dense racks of accelerator chips, high-speed networking, heavy power supply and liquid cooling. It differs from a traditional data center mainly in power per rack, which can be many times higher. Read more.
ASIC
An ASIC, or application-specific integrated circuit, is a chip designed for one task rather than general use. In AI, custom ASICs built by cloud companies run particular models efficiently and at lower power than a general-purpose GPU, though they are less flexible and costly to design. Read more.
Asset allocation
Asset allocation is the way an investor divides a portfolio among categories such as stocks, bonds, cash and alternatives. It is a main driver of how a portfolio behaves over time. Investors often set target percentages and periodically rebalance to keep the mix aligned with their goals and risk tolerance. Read more.
Assets under management (AUM)
Assets under management, or AUM, is the total market value of the assets a fund or firm manages for investors. For an ETF, AUM equals the shares outstanding multiplied by the net asset value per share. AUM changes with market moves and with investor flows into and out of the fund.
Assignment
Assignment is the event in which an option seller is required to fulfill the contract after the buyer exercises it. A call seller must deliver the asset at the strike, and a put seller must buy it. Index options settle in cash, so assignment means paying the difference. Read more.
Authorized participant
An authorized participant is a large financial institution permitted to create and redeem ETF shares directly with the fund. It delivers a basket of securities to receive new shares, or returns shares to receive securities. This process helps keep the market price of an ETF close to its net asset value. Read more.
B
Basket
A basket is the list of securities, and sometimes cash, that an authorized participant exchanges with an ETF to create or redeem shares. The fund publishes the basket each day. Its contents usually mirror the fund’s portfolio, though a fund may adjust it for tax or operational reasons. Read more.
Beta
Beta is a measure of how much an investment tends to move relative to a benchmark. A beta of 1.0 means it has moved in line with the benchmark historically, above 1.0 means larger swings, and below 1.0 means smaller ones. Beta is based on past data and can change. Read more.
Bid-ask spread
A bid-ask spread is the gap between the highest price a buyer offers (the bid) and the lowest price a seller accepts (the ask). It is a cost of trading, paid whenever an investor buys or sells at market prices. Spreads tend to be narrower for ETFs that are heavily traded and hold liquid assets. Read more.
Bill of materials
A bill of materials, or BOM, is the full list of components and subassemblies needed to build a finished product. Applied to an AI data center or a robot, it maps the physical inputs, such as power gear, cooling, chips and memory, and shows which industries receive the spending. Read more.
Bond ETF
A bond ETF is an exchange-traded fund that holds a portfolio of bonds and trades on an exchange like a stock. Unlike an individual bond, it has no maturity date, and it usually pays income monthly. Its price moves with interest rates, credit conditions and demand for the fund’s shares. Read more.
Buffer
A buffer is the stated range of index losses that a buffer ETF’s options are designed to absorb over an outcome period. If the buffer is 9 percent and the index falls 6 percent, the buffer absorbs that loss. Losses beyond the buffer are borne by the investor, and the cap limits gains. Read more.
Buffer ETF
A buffer ETF is a fund that uses options to pursue a defined outcome over a set period: the buffer absorbs a stated range of index losses, while a cap limits the gain. The outcome applies to investors who hold for the full period, and the fund’s share price can differ from the index along the way. Read more.
Buy-write
A buy-write is a strategy that buys a stock or index exposure and sells a call option on it at the same time. The investor collects the option premium in exchange for giving up gains above the strike price. It is the same position as a covered call, with the stock and option opened together. Read more.
C
Call option
A call option is a contract that gives its buyer the right, but not the obligation, to buy an asset at a set price, the strike, on or before an expiration date. The seller receives a premium and must deliver the asset if the option is exercised, which caps the seller’s upside. Read more.
Cap
A cap is the maximum return a buffer ETF can deliver over its outcome period, before fund fees. It exists because the options that create the buffer are paid for by selling away upside. The cap depends on option prices, interest rates and volatility at the start of the period, so it changes from one period to the next. Read more.
Capital expenditures (capex)
Capital expenditures, or capex, are payments a company makes to buy or build long-lived assets such as buildings, servers and equipment. In AI, the term usually refers to the large sums that cloud companies spend on data centers, chips and power, which flow to suppliers across the technology and industrial sectors. Read more.
Capital gains distribution
A capital gains distribution is a payment a fund makes to shareholders from net gains it realized by selling investments at a profit. It is taxed as long-term or short-term gain depending on how long the fund held the securities, not on how long the shareholder owned the fund. Read more.
Cash-secured put
A cash-secured put is a position in which an investor sells a put option and holds enough cash to buy the underlying asset at the strike price if it is assigned. The seller collects the premium. It is the building block of put-write strategies, which pursue income from option premium. Read more.
Circuit breaker
A circuit breaker is a rule that pauses trading on an exchange when a market index falls by a set percentage in a single day. U.S. rules use declines of 7, 13 and 20 percent in the S&P 500. Individual securities also have limit-up, limit-down bands that pause trading on sharp price moves. Read more.
Commodity ETF
A commodity ETF is a fund that gives exposure to raw materials such as gold, oil or agricultural products, either by holding the physical asset or by holding futures contracts. Futures-based funds can behave differently from the spot price because of the cost of rolling contracts from one month to the next. Read more.
Concentration
Concentration is the degree to which a portfolio’s value depends on a small number of holdings, sectors or themes. A concentrated fund can be more volatile than a broad one because a few positions drive results. Investors measure it by looking at top-ten weights and sector shares. Read more.
Core and satellite
Core and satellite is a portfolio approach that builds most of a portfolio around broad, low-cost holdings, the core, and adds smaller positions, the satellites, that pursue a specific theme, income pattern or risk profile. It lets an investor add focused exposure without letting it dominate the overall result. Read more.
Correlation
Correlation is a statistic from negative 1 to positive 1 that shows how closely two investments move together. A correlation near 1 means they tend to rise and fall together, while a value near 0 means little relationship. Lower correlation among holdings can reduce portfolio swings, though correlations tend to rise in sharp sell-offs. Read more.
Cost basis
Cost basis is the original amount an investor paid for a security, including commissions, and it is used to calculate gain or loss when the security is sold. Certain distributions classified as return of capital reduce cost basis for tax purposes, which can increase the taxable gain at sale. Read more.
Covered call
A covered call is an options strategy in which an investor owns an asset and sells a call option on it. The investor receives a premium but gives up gains above the strike price. Losses on the asset are cushioned only by the premium received, so the strategy still carries the asset’s downside risk. Read more.
Covered call ETF
A covered call ETF is a fund that holds stocks or an index and sells call options on them to generate income, which it typically pays out as monthly distributions. The income comes mainly from option premium. In exchange, the fund gives up some gains if the market rises sharply. Read more.
Creation and redemption
Creation and redemption is the process by which ETF shares are issued and retired. An authorized participant delivers a basket of securities to the fund and receives new shares, or returns shares for securities. It adjusts the supply of shares to demand and helps keep prices close to net asset value. Read more.
Creation unit
A creation unit is the block of ETF shares, often 10,000 to 50,000 shares, that an authorized participant can create or redeem with the fund. Individual investors do not trade creation units. They buy and sell ordinary shares on an exchange, and the creation unit mechanism works behind the scenes. Read more.
Critical minerals
Critical minerals are raw materials, such as copper, lithium, gallium and rare earth elements, that are essential to modern technology and energy systems and have supply risks. AI hardware, power equipment and defense systems depend on them, so mining and processing capacity can become a bottleneck. Read more.
D
Data center
A data center is a facility that houses computer servers, storage and networking equipment, along with the power and cooling systems that keep them running. Companies and cloud providers use data centers to run applications and store data, and AI has increased demand for larger, more power-dense sites. Read more.
Defense budget
A defense budget is the annual spending plan for a country’s military, divided into categories such as personnel, operations, procurement and research. The U.S. figure is set by Congress through authorization and appropriations laws. Shifts between categories show where governments are directing money. Read more.
Delta
Delta is a measure of how much an option’s price is expected to change for a one dollar move in the underlying asset. A call with a delta of 0.30 gains about 30 cents if the asset rises one dollar. Delta is also read as a rough estimate of the probability the option finishes in the money. Read more.
Direct indexing
Direct indexing is an approach in which an investor owns the individual stocks of an index in a separately managed account, instead of holding a fund. It allows customization and tax-loss harvesting on individual positions, but usually involves higher minimums and fees and more complexity than an ETF. Read more.
Discount
A discount occurs when an ETF’s market price is below its net asset value per share. It can appear when selling pressure is heavy or when underlying markets are closed or illiquid. Authorized participants usually act to narrow discounts, so large and lasting ones are uncommon for funds holding liquid assets. Read more.
Distribution
A distribution is a payment a fund makes to its shareholders from income, realized gains or other sources. ETFs may distribute monthly, quarterly or annually. Each distribution is reported by source for tax purposes, and the total can include ordinary income, capital gains and return of capital. Read more.
Distribution rate
A distribution rate is a fund’s recent payments expressed as an annualized percentage of its share price or net asset value. It shows what the fund has paid, not what it must pay in the future. It can include return of capital and gains, so it is not the same as the fund’s earnings yield. Read more.
Distribution yield
Distribution yield is the income a fund paid over a period, usually 12 months, divided by its share price. It describes cash paid out and ignores changes in share price. Investors compare it with total return, which adds price change to distributions, to judge overall results. Read more.
Diversification
Diversification is the practice of spreading investments across many holdings, sectors or asset types so that no single one drives the result. It can reduce the effect of any one position’s decline, but it does not guarantee a profit or prevent loss, especially when markets fall together. Read more.
Dollar-cost averaging
Dollar-cost averaging is the practice of investing a fixed dollar amount at regular intervals, regardless of price. The investor buys more shares when prices are low and fewer when they are high. It spreads out the timing of purchases but does not ensure a gain or remove the risk of loss. Read more.
Drawdown
A drawdown is the decline in an investment’s value from a peak to a later low, usually shown as a percentage. A maximum drawdown is the largest such decline over a period. Drawdowns help investors see how deep losses have run historically and how long recovery has taken. Read more.
Drone
A drone is an aircraft that flies without a human pilot on board, either remotely controlled or autonomous. Drones are used in defense, logistics, agriculture and inspection. Military interest has grown because drones can be built cheaply and in volume compared with crewed aircraft. Read more.
Dual directional
A dual directional buffer is a design that can earn a positive return when the index falls modestly as well as when it rises, up to a cap. The fall is converted to a gain within a stated range, and the buffer absorbs losses beyond that. Each design has its own limits. Read more.
E
Equal weight
Equal weight is an index method that gives every holding the same weight, such as 0.2 percent each in a 500-stock index, regardless of company size. It gives smaller companies more influence than market cap weighting does and requires frequent rebalancing back to equal weights. Read more.
ETF
An ETF, or exchange-traded fund, is an investment fund whose shares trade on a stock exchange throughout the day. It holds a basket of assets such as stocks, bonds or options. Investors buy and sell shares at market prices, which can differ slightly from the value of the fund’s holdings. Read more.
ETF closure
An ETF closure, also called liquidation, is the process of a fund shutting down. The fund sells its holdings and pays shareholders cash based on net asset value. Shareholders may owe tax on any gain, and can sell on the exchange before the final date. Read more.
Exchange-traded product
An exchange-traded product, or ETP, is any investment that trades on an exchange and tracks an asset, index or strategy. ETFs are the most common kind. Others include exchange-traded notes and commodity trusts, which have different legal structures, risks and tax treatment from funds registered under the Investment Company Act. Read more.
Expense ratio
An expense ratio is the annual fee a fund charges, shown as a percentage of assets. It is deducted from fund assets, so investors do not receive a bill. A 0.50 percent expense ratio costs $5 a year per $1,000 invested, and it reduces returns each year the fund is held. Read more.
Expiration
Expiration is the date on which an option contract ends. After that date, the option has no value unless it is in the money, in which case it is exercised or settled. Monthly, weekly and daily expirations exist, and shorter ones lose time value faster. Read more.
F
Financial advisor
A financial advisor is a professional who gives investment, planning or tax guidance to clients, often for a fee. Advisors use ETFs for diversification, income and risk management because of their trading flexibility and transparency. Advisors differ in credentials and duties, so investors should ask whether they act as fiduciaries. Read more.
FLEX options
FLEX options are exchange-listed option contracts with customizable terms, such as strike price and expiration date, that are cleared by the Options Clearing Corporation. Buffer ETFs use them to set exact outcome periods and levels. Because they are cleared centrally, they reduce counterparty risk compared with private options. Read more.
Floor
A floor is a limit on how much loss an investor can bear over an outcome period. With a 15 percent floor, the fund’s options absorb index declines beyond 15 percent, while the investor bears the first 15 percent. Floors usually come with lower caps than buffers, because the options that limit deeper losses cost more. Read more.
Foundry
A foundry is a factory that manufactures chips designed by other companies. Leading foundries operate extremely costly fabrication plants and use cutting-edge processes. Many AI chip designers do not make their own chips, so foundry capacity strongly shapes how many accelerators can be produced. Read more.
Fractional shares
Fractional shares are portions of a share that an investor can buy through a brokerage, such as 0.25 of an ETF share. They let investors put a specific dollar amount to work even when the price of one whole share is higher. Availability depends on the brokerage and on the fund. Read more.
Free float
Free float is the portion of a company’s shares available for public trading, excluding shares held by insiders, governments or other long-term holders. Many indexes weight companies by free-float market capitalization so that weights reflect shares investors can actually buy. Read more.
G
GPU
A GPU, or graphics processing unit, is a chip designed to perform many calculations at once. Originally built for graphics, GPUs became the main hardware for AI because training and running neural networks rely on large amounts of parallel math. They are a leading AI accelerator. Read more.
H
HBM
HBM, or high-bandwidth memory, is a type of computer memory made of stacked chips placed next to an accelerator to move data very fast. AI models need to read large amounts of data quickly, so HBM is a key part of AI hardware and has been in tight supply. Read more.
Hedge
A hedge is an investment made to offset potential losses in another holding. Options are commonly used to hedge, since a put gains when the asset falls. Hedging has a cost and can reduce gains, and it does not remove all risk or guarantee a result. Read more.
Humanoid robot
A humanoid robot is a machine built with a human-like body, including arms, legs and a head, so it can work in spaces designed for people. It combines actuators, batteries, sensors and AI models. Commercial use is early, and costs and reliability are still being worked out. Read more.
Hyperscaler
A hyperscaler is a very large cloud computing company that operates data centers at enormous scale, such as the biggest U.S. platform and cloud providers. Hyperscalers buy chips, servers and power equipment in huge volumes, so their capital spending plans influence demand across the AI supply chain. Read more.
I
Implied volatility
Implied volatility is the market’s expectation of how much an asset’s price may swing, as reflected in option prices. Higher implied volatility makes options more expensive, so sellers of options collect more premium. It is a forecast built into prices and does not guarantee future movement. Read more.
In the money
An option is in the money when exercising it would have value. A call is in the money when the asset’s price is above the strike, and a put is in the money when the price is below the strike. In-the-money options cost more because they already hold intrinsic value. Read more.
In-kind redemption
An in-kind redemption is a transaction in which an ETF gives securities, rather than cash, to an authorized participant in exchange for shares. Because the fund does not have to sell holdings to raise cash, it can reduce the capital gains the fund realizes, which is one reason ETFs often distribute fewer gains than mutual funds. Read more.
iNAV
The iNAV, or intraday net asset value, is an estimate of an ETF’s per-share value updated throughout the trading day, usually every 15 seconds. It uses live prices of the fund’s holdings. Investors compare it with the market price to see whether a fund trades at a premium or discount. Read more.
Index
An index is a rule-based list of securities that measures part of the market, such as the S&P 500 for large U.S. companies. Indexes are not investable directly. Funds track them by holding the same or similar securities, and the index sets the methodology for selection and weighting. Read more.
Index option
An index option is an option whose underlying asset is a market index, such as the S&P 500, rather than a single stock. Most are cash settled, meaning no shares change hands at expiration. Funds use them to build income and buffer strategies on broad index exposure. Read more.
Index reconstitution
Index reconstitution is the periodic review in which an index adds, removes or reweights its members based on its rules. Funds tracking the index trade to match. It can cause turnover, trading costs and, in taxable accounts, distributions, so funds with rules-based methodologies disclose their schedule. Read more.
Inference
Inference is the stage at which a trained AI model produces answers or predictions for users. Each query uses computing power, so inference demand grows with usage. It contrasts with training, a one-time and much larger effort to build the model, and it increasingly drives data center demand. Read more.
L
Laddering
Laddering is the practice of staggering investments so that they reset or mature at different times. With buffer ETFs, an investor holds funds with different outcome period start dates, so some reset every month or quarter. It spreads out timing and avoids relying on one entry date. Read more.
LEO
LEO stands for low Earth orbit, the region of space roughly 100 to 1,200 miles above the planet. Satellites there are closer to the ground than higher orbits, so signals travel faster, which suits broadband and imaging. They move quickly, so large constellations are needed for continuous coverage. Read more.
Leverage
Leverage is the use of borrowing or derivatives to increase exposure beyond the amount invested. Leveraged ETFs seek a multiple of an index’s daily return, which magnifies both gains and losses. Because they reset daily, their results over longer periods can differ sharply from the stated multiple. Read more.
Limit order
A limit order is an instruction to buy or sell a security only at a specified price or better. It gives the investor control over the price paid, but it may not execute if the market never reaches that price. Limit orders are commonly used for ETFs to avoid wide spreads. Read more.
Liquid cooling
Liquid cooling is a way of removing heat from computer chips by circulating coolant through cold plates or immersion tanks rather than blowing air. It is far more effective at high power levels, so it has become standard for dense AI racks that draw well over 100 kilowatts. Read more.
Liquidity
Liquidity is how quickly an asset can be bought or sold at a price close to its current value. An ETF’s liquidity depends on the trading volume of its shares and on how easily its underlying holdings can be traded. High liquidity usually means narrower bid-ask spreads. Read more.
Long-term capital gain
A long-term capital gain is profit from selling an asset held more than one year, and in the United States it is generally taxed at lower rates than ordinary income. For funds, distributions of gains the fund held over a year are long-term, no matter how long the shareholder owned shares. Read more.
M
Market cap weight
Market cap weight is an index method that sets each holding’s weight in proportion to its market capitalization, the share price times shares outstanding. Larger companies get larger weights. The S&P 500 and Nasdaq-100 are both capitalization weighted, so their results lean heavily on their biggest holdings. Read more.
Market maker
A market maker is a firm that continuously quotes prices to buy and sell a security, earning the bid-ask spread. In ETFs, market makers keep shares liquid on the exchange and, working with authorized participants, help keep share prices near net asset value. Read more.
Market order
A market order is an instruction to buy or sell immediately at the current market price. It almost always executes, but the price can differ from what an investor expected, especially in fast markets or for thinly traded ETFs, where a limit order may be a more careful choice. Read more.
Megawatt
A megawatt is a unit of power equal to one million watts. Data center capacity is often measured in megawatts of electricity the site can draw. A gigawatt is 1,000 megawatts, enough to supply a large city, and gigawatt-scale AI campuses are now being planned. Read more.
Moneyness
Moneyness describes where an option’s strike price sits relative to the current price of the underlying asset. An option can be in the money, at the money or out of the money. Moneyness helps determine an option’s price, its sensitivity to price moves and its chance of finishing with value. Read more.
Monthly distribution
A monthly distribution is a payment a fund makes once a month from its income, which may include option premium, interest and realized gains. The amount can vary from month to month. Funds that pay monthly can suit investors who want regular cash flow, though payment levels are not fixed. Read more.
Munitions
Munitions are weapons and ammunition such as shells, missiles and guided bombs. Production capacity depends on specialized plants, energetic materials and skilled workers, and it expands slowly. Recent conflicts have drawn down stocks, so governments are funding larger production lines. Read more.
N
Nasdaq-100
The Nasdaq-100 is an index of the 100 largest non-financial companies listed on the Nasdaq exchange, weighted by market capitalization. It leans toward technology and growth companies, so it can be more volatile than the S&P 500. Many ETFs and options track it. Read more.
NAV
NAV, or net asset value, is the per-share value of a fund, calculated by taking the value of all its holdings, subtracting liabilities and dividing by shares outstanding. ETFs calculate NAV once a day after the market closes, while the market price changes continuously during trading. Read more.
NDAA
The NDAA is the National Defense Authorization Act, a U.S. law passed each year that sets policy for the Department of Defense and authorizes spending levels. It directs programs, procurement rules and priorities, so it is a guide to where defense spending may be directed. Appropriations bills provide the actual funding. Read more.
Notional value
Notional value is the total value of the asset that an option or derivative contract controls. One S&P 500 option on an index at 5,000 controls $500,000 of exposure at a $100 multiplier. Funds use notional value to size how much of their portfolio an options position covers. Read more.
O
Optical transceiver
An optical transceiver is a module that converts electrical signals into light and back, so data can move through fiber optic cables. AI clusters use huge numbers of them to link switches and servers at high speed, and demand for faster versions has grown with accelerator counts. Read more.
Option income ETF
An option income ETF is a fund that sells options to earn premium and pays it out as distributions. Strategies include covered calls, put-writing and buy-writes. Income depends on market volatility, and in exchange the fund usually gives up some of the upside of its underlying index. Read more.
Option premium
Option premium is the price a buyer pays to a seller for an option contract. It is made up of intrinsic value and time value, and it rises with higher volatility and longer time to expiration. For income funds, the premium collected is the main source of distributions. Read more.
Options Clearing Corporation
The Options Clearing Corporation, or OCC, is the clearinghouse for U.S. listed options. It stands between buyers and sellers of every contract and guarantees performance, which reduces counterparty risk. Buffer ETFs rely on exchange-listed options cleared by the OCC. Read more.
Ordinary income
Ordinary income is income taxed at an individual’s regular federal tax rates, as opposed to the lower rates for long-term capital gains. Interest and many distributions from funds that earn option premium are reported as ordinary income or short-term gains, so after-tax results depend on the investor’s bracket. Read more.
Out of the money
An option is out of the money when exercising it would have no immediate value. A call is out of the money when the asset’s price is below the strike, and a put is out of the money when the price is above it. These options cost less and are more likely to expire worthless. Read more.
Outcome period
An outcome period is the span of time over which a buffer ETF’s stated buffer and cap apply, often one month or one year. The outcomes are designed for investors who buy at the start and hold to the end. Buying or selling in the middle can produce different results. Read more.
Overlap
Overlap is the share of holdings that two or more funds have in common. High overlap means that owning both adds little diversification, because the same stocks drive both funds. Investors can check overlap by comparing top holdings and weights before combining funds in one portfolio. Read more.
Overwrite
An overwrite is the practice of selling call options against stock holdings. The overwrite percentage is the share of the portfolio that the options cover. A higher overwrite brings in more premium but caps more of the upside, while a partial overwrite keeps some exposure to gains. Read more.
P
Partial overwrite
A partial overwrite is a covered call approach that sells call options on only part of the portfolio, such as half. The investor collects some premium while leaving the remainder free to participate in rising markets. It trades a smaller amount of income for more upside participation. Read more.
Passive ETF
A passive ETF is a fund that aims to match the holdings and returns of an index rather than beat it. Because it follows rules rather than manager judgment, it typically has a lower expense ratio and lower turnover. It still carries the full market risk of the index it follows. Read more.
Physical AI
Physical AI is artificial intelligence that perceives and acts in the physical world through machines such as robots, drones and autonomous vehicles. It combines sensors, computing chips, software models and actuators. It differs from software-only AI because it must handle real-world conditions and safety demands. Read more.
Premium
An ETF premium occurs when its market price is above its net asset value per share. It can appear when demand is strong or when underlying markets are closed. Authorized participants typically create shares to narrow a premium. Do not confuse it with option premium, which is the price of an option. Read more.
Price return
Price return is the change in an investment’s price over a period, not counting any income paid out. The S&P 500 price index, for example, excludes distributions. Buffer ETFs usually describe outcomes against the price return of an index, because the options reference index price levels. Read more.
Primary market
The primary market is where new ETF shares are created and redeemed, between the fund and authorized participants. Individual investors do not trade here. It works alongside the secondary market, where investors buy and sell existing shares, and it keeps the supply of shares in line with demand. Read more.
Procurement
Procurement is the process by which a government buys goods and services, such as aircraft, munitions and electronics. In the U.S. defense budget, procurement is the account that pays for buying weapons and equipment, separate from research, operations and personnel, and it tends to follow multiyear contracts. Read more.
Put option
A put option is a contract that gives its buyer the right, but not the obligation, to sell an asset at a set strike price before expiration. The seller collects a premium and must buy the asset at the strike if assigned. Put-write strategies sell puts to earn income. Read more.
Put-write
A put-write is a strategy that sells put options, usually on an index, and holds cash or Treasury bills to cover the obligation. It earns premium income and carries exposure to index declines beyond the premium collected. Put-write indexes track the results of this approach. Read more.
Q
Qualified dividend
A qualified dividend is a payout from a domestic or eligible foreign corporation that meets IRS holding period rules and is taxed at long-term capital gain rates. Funds report the qualified portion on Form 1099-DIV. Distributions from option premium generally do not qualify, so a fund’s mix affects its tax treatment. Read more.
R
Rack density
Rack density is the amount of electrical power drawn by a single server rack, measured in kilowatts. Traditional enterprise racks draw around 10 kilowatts, while AI racks can exceed 100. Higher density concentrates more computing in less space but demands more power delivery and cooling. Read more.
RDT&E
RDT&E stands for research, development, test and evaluation, the part of the U.S. defense budget that funds new technology from early research through testing before a system is bought in volume. It is a separate account from procurement and signals which capabilities the government is developing for the future. Read more.
Realized volatility
Realized volatility is the amount an asset’s price actually moved over a past period, usually measured as the annualized standard deviation of daily returns. It looks backward, while implied volatility looks forward. When implied volatility runs above realized, option sellers have tended to collect more than the moves they experienced. Read more.
Rebalancing
Rebalancing is the act of buying and selling holdings to bring a portfolio back to its target weights. Without it, assets that have risen grow into a larger share of the portfolio. Investors rebalance on a schedule or when weights drift past a set threshold, and it may trigger taxes in taxable accounts. Read more.
Remaining buffer
Remaining buffer is the amount of the buffer still available to absorb losses at a given point in an outcome period, for an investor buying the fund then. It differs from the original buffer once the index has moved or the fund’s price has changed. Fund providers publish it on their websites. Read more.
Remaining cap
Remaining cap is the additional upside an investor can still earn through the end of the outcome period if buying at the current price. It shrinks as the fund’s share price rises toward the cap and grows if the price falls. Fund providers publish it, and it can differ from the original cap. Read more.
Reset date
A reset date is the day a buffer ETF’s outcome period ends and a new one begins, with a new buffer, a new cap and fresh options. Investors who hold through the reset start the next period at the then-current index level. Monthly buffer funds reset every month and annual funds reset once a year. Read more.
Return of capital
Return of capital is a tax classification for part of a distribution that is not paid out of a fund’s earnings and profits. It is not taxed when received, but it lowers the shareholder’s cost basis, which can raise the gain or lower the loss when shares are sold. It is reported on Form 1099-DIV. Read more.
Reusable rocket
A reusable rocket is a launch vehicle whose parts, usually the first stage, return to Earth and fly again. Reuse has cut the cost of reaching orbit substantially, which makes large satellite constellations and other space activities more economical. Read more.
Risk tolerance
Risk tolerance is the amount of investment loss, and the volatility along the way, that an investor can accept emotionally and financially in pursuit of a goal. It depends on time horizon, income needs and temperament. Matching holdings to risk tolerance helps investors stay with a plan during market declines. Read more.
Roth IRA
A Roth IRA is a retirement account funded with after-tax dollars in which qualified withdrawals are tax-free. ETFs can be held inside it, and distributions and gains are not taxed each year while they stay in the account. Contribution limits and income eligibility rules apply. Read more.
S
S&P 100
The S&P 100 is an index of 100 large, established U.S. companies drawn from the S&P 500, weighted by market capitalization. It is more concentrated than the S&P 500 and tilts toward the largest companies. Options on the index are listed, which allows funds to build option strategies on it.
S&P 500
The S&P 500 is an index of 500 large U.S. companies, weighted by market capitalization, and it is widely used as a measure of the U.S. stock market. It covers roughly 80 percent of U.S. equity market value. Many ETFs track it, and its listed options underlie many income and buffer strategies. Read more.
Satellite constellation
A satellite constellation is a group of satellites that work together, often in low Earth orbit, to provide coverage across wide areas. Broadband constellations use hundreds or thousands of satellites so that at least one is always overhead, which creates demand for manufacturing and launch. Read more.
Secondary market
The secondary market is where investors trade existing ETF shares with each other on an exchange, as opposed to the primary market, where shares are created and redeemed. Prices are set by supply and demand from buyers and sellers, and market makers keep quotes available throughout the trading day. Read more.
Section 1256 contract
A Section 1256 contract is a tax category under the Internal Revenue Code that includes listed broad-based index options. Gains are marked to market at year end and treated as 60 percent long-term and 40 percent short-term, whatever the holding period. How it applies to a fund depends on the fund’s structure. Read more.
Section 19(a) notice
A Section 19(a) notice is a written statement that a fund must send with a distribution when part of it comes from a source other than net investment income, such as capital gains or return of capital. It is an estimate of the sources and is finalized later on Form 1099-DIV. Read more.
Sector
A sector is a group of companies in the same broad line of business, such as technology, health care or energy. Standard classification systems sort companies into about eleven sectors. Looking at sector weights shows how much a fund depends on one part of the economy. Read more.
Semiconductor
A semiconductor is a material, usually silicon, that conducts electricity under controlled conditions, and the word also refers to the chips built from it. Chips power computers, phones, cars and AI systems. Making them involves design, manufacturing at a foundry, packaging and testing across several countries. Read more.
Sequence of returns risk
Sequence of returns risk is the danger that poor market returns early in retirement, while withdrawals are being taken, cause lasting damage to a portfolio. The same average return can leave very different balances depending on order. It is one reason retirees pay attention to income sources and downside. Read more.
Short-term capital gain
A short-term capital gain is profit from selling an asset held for one year or less, and it is taxed at ordinary income rates. Funds that trade often or earn option income can produce short-term gains, which they distribute to shareholders, so the tax effect depends on the investor’s bracket. Read more.
Single-stock ETF
A single-stock ETF is a fund built around one company’s shares, often using options or leverage to seek a defined result. Because it depends on a single company, it can be much more volatile than a diversified fund. Some single-stock funds pursue income by selling calls on that stock. Read more.
SMR
An SMR, or small modular reactor, is a nuclear power plant that is smaller than a traditional reactor and designed to be built in factory-made modules. Data center developers have shown interest in SMRs as a steady power source, though most designs remain in development or early licensing. Read more.
Space economy
The space economy is the set of industries that build, launch and use satellites and related services, including communications, Earth observation, navigation and launch. It includes government and commercial activity. Lower launch costs from reusable rockets have widened what companies can do in orbit. Read more.
Strike price
A strike price is the fixed price at which the holder of an option can buy (for a call) or sell (for a put) the underlying asset. Choosing a strike is a central decision in option income strategies: a strike closer to the market price earns more premium but gives up more upside. Read more.
Supercycle
A supercycle is a long period, often a decade or more, in which demand for a category of assets or industries rises well above its historical trend, usually driven by a structural change such as industrialization or AI. It differs from an ordinary business cycle in its length and scale. Read more.
Systematic withdrawal
A systematic withdrawal is a plan to take set amounts from a portfolio at regular intervals, such as monthly. Withdrawals may come from distributions, sales of shares or both. The plan’s durability depends on withdrawal rate, market returns and the order of those returns. Read more.
T
T+1
T+1 is the U.S. settlement cycle in which a trade of stocks and ETFs settles one business day after the trade date. It replaced T+2 in May 2024. After settlement, the buyer officially owns the shares and the seller receives the cash, which affects when funds are available. Read more.
Tax lot
A tax lot is a block of shares bought at one time and price. Each lot has its own cost basis and holding period, which determine the gain or loss when sold. Brokers let investors choose which lots to sell, such as the highest cost first, to manage taxes. Read more.
Tax-loss harvesting
Tax-loss harvesting is the practice of selling an investment at a loss to offset taxable gains, and then reinvesting in a similar but not identical holding. The wash sale rule limits it. Losses can offset gains and up to $3,000 of ordinary income each year, with extra losses carried forward. Read more.
Thematic ETF
A thematic ETF is a fund that invests in companies tied to a specific long-term trend, such as artificial intelligence, robotics or space, rather than a broad market. Because holdings cluster around one idea, results can be more volatile than a broad fund, and the methodology determines which companies qualify. Read more.
Theta
Theta is a measure of how much an option loses in value each day as expiration gets closer, all else being equal. It is also called time decay. Option sellers benefit from theta because the options they sold tend to lose value as time passes, which is part of how income strategies earn premium. Read more.
Time horizon
Time horizon is the length of time an investor expects to hold an investment before needing the money. A longer horizon usually allows an investor to ride out market declines, while a shorter one calls for more attention to volatility. Buffer funds are built around specific outcome periods. Read more.
Total return
Total return is the full gain or loss on an investment over a period, including both price change and income such as distributions, assuming income is reinvested. It gives a complete picture of performance, and is a fairer comparison between funds than yield or price alone. Read more.
Tracking difference
Tracking difference is the gap between an ETF’s return and the return of its index over a period. It is the result of fees, trading costs and how closely the fund holds the index. A negative tracking difference means the fund lagged the index, typically by about the expense ratio. Read more.
Tracking error
Tracking error is the variation in the daily or monthly gap between an ETF’s return and its index return, usually shown as a standard deviation. A low tracking error means the fund follows its index consistently. It differs from tracking difference, which measures the total gap over a period. Read more.
Trailing 12-month yield
Trailing 12-month yield is the sum of a fund’s distributions over the past year divided by its current share price. It reports what was paid, so it can overstate or understate future income. It is simple to calculate, but for funds with changing payments it is a backward-looking measure. Read more.
Training
Training is the process of building an AI model by feeding it large amounts of data and adjusting its parameters until it performs well. It requires thousands of accelerators running for weeks or months. Training is costly and happens before a model is released, while inference happens when people use it. Read more.
Transformer
A transformer is an electrical device that raises or lowers voltage so power can travel over lines and be used safely. Large power transformers are custom built, take a long time to make and are in short supply, which makes them a bottleneck for connecting new data centers to the grid. Read more.
U
Unrealized gain
An unrealized gain is an increase in an investment’s value that has not been locked in by selling. It is a paper gain, not taxed until the investment is sold. ETFs can pass fewer gains to shareholders because in-kind redemptions help move low-basis holdings out of the portfolio. Read more.
Upside participation
Upside participation is the share of an index’s gain that an investor receives over an outcome period. A buffer ETF may deliver full participation up to its cap and none above it. Some designs, such as accelerated buffers, give more than 100 percent participation up to a lower cap. Read more.
V
VIX
The VIX is a volatility index that measures the market’s expected 30-day volatility in the S&P 500, calculated from option prices. Higher readings indicate larger expected swings. It is often called a fear gauge, and because option premiums rise with volatility, it is closely followed by income investors. Read more.
Volatility
Volatility is the degree to which an asset’s price moves up and down over time, usually measured as the standard deviation of returns. Higher volatility means larger and more frequent swings. It is a measure of risk, though it captures both gains and losses, and it affects option prices. Read more.
VXN
The VXN is a volatility index that measures expected 30-day volatility in the Nasdaq-100, based on option prices. It tends to run higher than the VIX because the Nasdaq-100 is more concentrated in growth companies, and it influences the premium available to option sellers on that index. Read more.
W
Wash sale
A wash sale occurs when an investor sells a security at a loss and buys the same or a substantially identical one within 30 days before or after the sale. The IRS disallows the loss for that tax year and adds it to the cost basis of the new purchase. Read more.
0 to 9
0DTE option
A 0DTE option is an option contract that expires the same day it is traded, with zero days to expiration. Because so little time remains, its price moves quickly with the underlying index. Some option income strategies sell very short-dated options to collect premium frequently, which also means results can change sharply within a single session. Read more.
1099-DIV
A 1099-DIV is the tax form that funds and brokers send to shareholders each year to report distributions received. It breaks them into categories such as ordinary income, qualified dividend amounts, capital gains distributions and return of capital. It is usually issued in January or February, and can arrive later. Read more.
30-day SEC yield
The 30-day SEC yield is a standardized measure of a fund’s income, calculated from net investment income over the latest 30 days and annualized using a formula set by the Securities and Exchange Commission. It lets investors compare funds on equal terms, but it may differ from the distribution rate, which reflects what was actually paid. Read more.
Cite this page: Adam Patti, "ETF Glossary: Plain-English Definitions of ETF Terms," VistaShares, updated November 13, 2026, https://www.vistashares.com/etf-glossary/. Plain-text version.
This material is provided for educational purposes only. It is not investment, legal or tax advice, and it is not a recommendation or an offer to buy or sell any security. Information is drawn from sources believed to be reliable as of the date of publication, is not guaranteed as to accuracy or completeness, and may change without notice. Definitions are general and simplified, and tax treatment depends on individual circumstances. Investors should consult their own financial, legal and tax professionals before making investment decisions. Investments involve risk, including possible loss of principal. Option strategies, buffers and thematic investments carry specific risks and may limit gains. References to companies, governments or programs are for illustration only.