Glossary
This glossary defines essential terminology used throughout market mechanics, from order-routing infrastructure to settlement procedures and participant roles. Whether you're learning how limit orders work, why short squeezes occur, or how regulatory safeguards protect investors, these terms form the foundation of market literacy. Each entry includes a definition and practical context to help you understand both individual concepts and their relationships within the broader trading ecosystem.
ACATS
Automated Customer Account Transfer Service — the industry-standard protocol for moving customer assets between brokers.
ACATS streamlines the process of transferring stocks, cash, and other holdings from one brokerage to another without requiring the customer to sell positions. The system, operated by FINRA, coordinates with both the transferring and receiving brokers to ensure a complete handoff of positions and regulatory records. A customer moving their IRA holdings from one custodian to another would rely on ACATS to handle the transfer within the typical 7-10 business day window, preserving the account's tax-deferred status throughout.
See also: Settlement, Clearinghouse.
ADR
American Depositary Receipt — a US-tradable security representing ownership of shares in a foreign company.
ADRs allow investors to hold and trade foreign stocks on US exchanges without opening accounts abroad or dealing in foreign currency directly. A European investor owning Nestle stock can sponsor an ADR, and a US investor can then buy and sell that ADR (ticker: NSRGY) on Nasdaq, with the underlying shares held in custody by a bank. ADRs simplify foreign investment by consolidating currency conversion, custody, and dividend collection into a single instrument.
See also: Cross-listing.
After-Hours Trading
Trading that occurs outside regular market hours (4:00 PM to 9:30 AM ET), typically on alternative trading systems with lower volume and wider spreads.
After-hours sessions allow traders to respond to earnings announcements, economic data, or international events before the next market open. A stock might gap significantly from its 4:00 PM close to the next 9:30 AM open if major news breaks in the evening; traders in after-hours markets begin repricing the security in real-time. However, after-hours spreads are often two to five times wider than daytime spreads, and liquidity is thin, making large orders difficult to execute at predictable prices.
See also: Market Hours, Spread, Liquidity.
AON
All-or-None — a time-in-force instruction that cancels the order if the broker cannot fill it in its entirety.
An AON order of 1,000 shares will either execute for all 1,000 shares or not at all; the broker will not execute a partial fill. This is useful for traders managing positions that require a specific total size, or for whom the round-lot economics matter. An institutional investor entering a large block trade might use AON to avoid ending up with an awkward fractional position.
See also: Time-in-Force, Fill-or-Kill.
ATS
Alternative Trading System — a non-exchange venue (such as a dark pool) that matches buyers and sellers outside the public order book.
ATSs are regulated by the SEC but are not required to display their order books in real-time, giving them a privacy advantage over lit exchanges. Bloomberg Tradebook, Instinet, and various bank-operated pools operate as ATSs, capturing volume from large institutions seeking to minimize market impact. The rise of ATS volume — now routinely 20–30% of US equity volume — reflects institutional demand for anonymity and pre-negotiated pricing.
See also: Dark Pool, Lit Market.
Best Execution
The broker's regulatory obligation to route trades to venues offering the most favorable price and execution quality for the customer.
Brokers must document and demonstrate that they are not simply routing all trades to the venue paying them the highest rebate (payment for order flow), but instead prioritizing the customer's benefit. If Nasdaq is offering a $0.01 better price than a broker-owned dark pool, best execution requires routing to Nasdaq. Violations can result in fines and disgorgement of ill-gotten rebates, as regulators view best execution as fundamental to fair markets.
See also: PFOF, Market Maker.
BBO
Best Bid-Offer — the highest bid and lowest ask across all trading venues in real-time.
The BBO is the "true" top-of-book price that consolidates activity across all lit markets. If Nasdaq's top bid is $50.25 and the NYSE's top ask is $50.26, the BBO is 50.25 bid, 50.26 ask. Regulators use BBO benchmarks to evaluate market quality and to police insider trading (a trader should not consistently buy ahead of large NBBO-crossing orders, a sign of front-running).
See also: NBBO, Order Book.
Bid-Ask Spread
The difference between the highest price a buyer is willing to pay (bid) and the lowest price a seller is willing to accept (ask).
A spread of $0.05 on a $100 stock (S&P 500 index component) is tight; a spread of $0.50 on a thinly-traded small-cap stock is wide. Tighter spreads reduce trading costs for all market participants. Market makers earn their profit from the spread, and competition among market makers on major exchanges drives spreads down to a few cents.
See also: Market Maker, Liquidity.
Buyback
A corporation's repurchase of its own outstanding shares, reducing share count and often increasing per-share earnings.
A company with $1 billion in annual net income and 100 million shares outstanding has earnings per share (EPS) of $10. If it buys back 10 million shares and earnings remain flat, the new EPS becomes $10.10 (on 90 million shares). Buybacks are a popular capital allocation tool, especially when management believes the stock is undervalued; however, critics argue they can prioritize short-term stock price appreciation over long-term investment.
See also: Dividend.
Circuit Breaker
An automatic trading halt triggered when a broad market index (S&P 500, Nasdaq-100, or individual stock) falls or rises by a specified percentage within a single trading day.
The first circuit breaker halts all US stock trading for 15 minutes if the S&P 500 drops 7% before 3:25 PM; a second level triggers at 13% decline; a third at 20% decline (which halts trading for the rest of the day). Circuit breakers were introduced after the 1987 crash and have prevented panicked liquidation cascades several times since, including in March 2020 and again in 2024. They give market participants time to reassess and prevent algorithmic cascade failures.
See also: Market Volatility.
Clearinghouse
A central institution that interposes itself between buyer and seller, guaranteeing each side's settlement obligation and managing counterparty risk.
The Options Clearing Corporation (OCC) acts as clearinghouse for all US equity options; DTCC's subsidiary NSCC does the same for stocks. When you buy 100 shares, you owe the cash and the seller is owed the cash — but both parties deal with the clearinghouse rather than each other directly. If a broker fails mid-trade, the clearinghouse ensures the other party still receives their shares or cash, eliminating bilateral credit risk.
See also: DTCC, NSCC, Settlement.
Co-Location
Placing a trader's servers in the same physical facility as a stock exchange's matching engine to reduce latency and gain a speed advantage.
An exchange charges a co-location fee (often $1,000–5,000 per month) to place your servers in their data center. The latency benefit is microseconds — but microseconds allow high-frequency traders to see and react to order flow before it reaches the broader market. Regulators tolerate co-location as a service available to all participants, but it reinforces the speed advantage of well-funded trading firms.
See also: HFT, Latency Arbitrage.
Cross-Listing
The simultaneous listing of a company's shares on multiple exchanges in different countries or regions.
Samsung Electronics is listed on both the Korea Exchange and the NYSE (as an ADR). Dual listing expands the investor base, improves liquidity, and reduces the cost of capital. However, the company must comply with both sets of regulatory requirements, including financial reporting standards and board governance rules.
Dark Pool
A private trading venue that does not display its order book publicly, allowing large investors to trade anonymously and with reduced market impact.
Citadel Securities, Virtu Financial, and bank-owned pools like JPMorgan's dark pool execute a combined 10–15% of US equity volume off the public order book. A mutual fund wanting to buy 5 million shares of Apple without moving the price upward by 2% might split the order across dark pools, taking 500,000 shares from each of ten pools at slightly varying prices. Dark pools improve execution for large trades but have been criticized for enabling unfair speed advantages and information leakage.
See also: ATS, PFOF, Lit Market.
Day Order
An order that cancels automatically if not filled by the close of the trading day.
A buy order placed at 10:00 AM that remains unfilled at 4:00 PM close is automatically cancelled; the order does not carry over to the next day. Most brokers default to day orders to prevent old limit orders from executing unexpectedly days or weeks later. A trader must explicitly request GTC (good-till-cancelled) to keep an order active across multiple days.
See also: GTC, Time-in-Force.
Depth of Market
The volume of orders available at different price levels above and below the current bid-ask, indicating how much volume can be absorbed at different prices without a large price move.
A stock with "deep" order book might have 100,000 shares bid at $50.00 and 150,000 shares offered at $50.01, indicating strong two-sided interest. A stock with "thin" depth might have only 10,000 shares on each side, meaning a large market order would have to accept worse prices to fill completely. Deep markets are associated with high-liquidity stocks; thin markets are typical of illiquid micro-caps.
See also: Order Book, Liquidity, Level 2.
Direct Listing
A process allowing existing shareholders to sell their shares publicly without the company raising new capital, bypassing the traditional IPO.
Spotify and Slack both went public via direct listing, allowing employees and early investors to sell immediately without the lock-up period typical of IPOs. The company does not raise new capital (unlike an IPO), but it gains a public listing and enables shareholder liquidity. Direct listings have become a popular alternative to IPOs, especially for late-stage private companies with sufficient scale.
See also: IPO, Secondary Offering.
DTCC
The Depository Trust and Clearing Corporation — the central securities depository and clearing organization for US equities and fixed income.
The DTCC holds nearly all US securities in electronic form (via subsidiary DTC, the Depository Trust Company), operates settlement infrastructure (via NSCC), and processes trillions of dollars in transactions annually. When you buy a stock, the DTCC ensures the seller's shares are in a vault and available to transfer to you, eliminating the physical delivery risk that plagued markets decades ago.
See also: Clearinghouse, Settlement, NSCC.
Fail-to-Deliver
Failure by a seller to deliver securities to a buyer by settlement date, creating a failed trade obligation.
If a short seller fails to deliver borrowed shares on settlement day, the trade is marked as "fail" and the buyer does not receive the shares, creating a claims liability. Persistent fail-to-delivers can indicate naked short selling or operational breakdowns; the SEC monitors fail levels and can impose restrictions on heavily-failed stocks. In 2008, major failures occurred due to bankruptcy-related disruptions; in 2021, heavily-shorted meme stocks accumulated substantial fails.
See also: Short Selling, Settlement, T+1.
Fill-or-Kill
An instruction to cancel an order immediately if it cannot be executed in full right away, as opposed to leaving a partial fill to rest.
A trader entering a 100,000-share FOK order wants either 100,000 shares now or nothing; if only 80,000 shares are available to buy at the limit price, the entire order is cancelled. This is more aggressive than AON, which can wait for a period for the full size to become available. HFT firms often use FOK orders to test the market without leaving resting orders that others can analyze.
See also: AON, Time-in-Force.
Follow-On Offering
A secondary offering of shares by a company that is already public, issued after its IPO.
A company might issue an IPO at $20 per share, raising $200 million for expansion. Six months later, the stock is trading at $35, and the company conducts a follow-on offering at $34.50, raising another $150 million. Follow-ons are dilutive to existing shareholders (more shares outstanding) but provide fresh capital without debt. Also called a secondary offering.
See also: IPO, Direct Listing, Dilution.
Free-Riding
Purchasing a security with unsettled cash and selling it before settlement is complete, avoiding payment.
Under Reg T, an investor in a cash account must wait for settlement before using proceeds from a sale to buy another security. If an investor sells stock and immediately uses that cash to buy another stock the same day, before settlement clears, they are "free-riding" on the float. Brokers can restrict or ban repeat offenders from margin and day trading.
See also: Reg T, Settlement.
GTC
Good-Till-Cancelled — a time-in-force designation that keeps an order active across multiple trading days until it is manually cancelled or executed.
A limit order to buy 100 shares of Apple at $140, entered as GTC, remains active day after day until either it fills at $140 or the trader cancels it. Some brokers auto-cancel GTC orders after 30 or 90 days; others require manual renewal. GTC orders can execute unexpectedly when earnings or news moves the stock into the limit price.
See also: Day Order, Time-in-Force.
Hard-to-Borrow
A stock that is expensive or impossible to borrow for short selling due to limited shares available in the lending market.
A newly-public small-cap with limited float might have an annualized borrow cost of 50% or higher; borrowing 1,000 shares for a month costs hundreds of dollars. Some stocks become so hard to borrow that brokers simply block short selling entirely. Meme stocks and heavily-shorted names frequently enter hard-to-borrow status, increasing the friction and cost of establishing short positions.
See also: Short Selling, Short Interest.
HFT
High-Frequency Trading — automated trading using powerful computers and minimal latency to profit from tiny price discrepancies and order flow patterns.
HFT firms like Virtu Financial and Citadel Securities execute millions of orders per day, often holding positions for milliseconds or less. They make profit on the bid-ask spread, arbitrage between exchanges, and statistical patterns in order flow. HFT is controversial: critics argue it benefits from informational advantages and creates systemic fragility; supporters note that HFT has made markets tighter and more efficient.
See also: Co-Location, Latency Arbitrage, Market Maker.
IEX
Investors Exchange — an alternative stock exchange designed to minimize unfair speed advantages and protect retail investors and passive managers from predatory trading.
IEX introduced a 350-microsecond "speed bump" that delays order book updates, preventing high-frequency traders from reacting faster than traditional market participants. IEX also prohibits market rebates (payment for order flow) and focuses on attracting buy-side flow from index funds and institutional investors. Despite regulatory backing, IEX has captured less than 2% of US equity volume.
IOC
Immediate-or-Cancel — a time-in-force that fills what is immediately available and cancels the rest, as opposed to leaving a resting order.
An IOC order to buy 10,000 shares will execute whatever quantity is offered at the limit price right now, and any unfilled portion is immediately cancelled. IOC is faster than FOK because it accepts partial fills; unlike GTC, it does not leave a resting order. Traders use IOC to test the market quickly without committing to hold an order.
See also: Fill-or-Kill, Time-in-Force.
IPO
Initial Public Offering — the process by which a private company becomes publicly-traded, selling shares to the public for the first time.
A venture-backed startup with 10 million shares outstanding files an S-1 registration statement, prices the IPO at $20 per share, and sells 2 million new shares to the public, raising $40 million. Existing shareholders (founders, investors) can also sell shares in the IPO; this is called the "secondary" component. IPOs are heavily regulated, require a roadshow presentation to institutional investors, and typically underpriced by 10–20% relative to first-day trading.
See also: Direct Listing, Secondary Offering.
Latency Arbitrage
Exploiting the speed advantage of co-located servers to detect and trade on information before it reaches other market participants.
A high-frequency trader with a server in an exchange data center can see an order flow pattern 50 microseconds before competing traders in other locations receive the same information. The HFT buys the stock a fraction of a second before a large institutional order moves the price up, then sells into the wave of demand. Regulators tolerate some latency arbitrage (it requires legitimate infrastructure investment) but prohibit more predatory variants like "spoofing" (fake orders).
See also: Co-Location, HFT, Front-Running.
Level 2
Real-time display of all bids and asks beyond the top bid-ask, showing the full order book depth at each price level.
A Level 1 quote shows only the top bid ($50.25) and top ask ($50.26). Level 2 reveals that there are 5,000 shares bid at $50.24, 12,000 at $50.23, and so on, as well as offers stacked from $50.27 upward. Professional traders use Level 2 to gauge how easily a large order can be filled and to spot attempted manipulation (e.g., spoofing, where traders place large fake orders to create a false appearance of support).
See also: Order Book, Depth of Market.
Limit Order
An instruction to buy or sell a security only at a specified price or better.
A limit order to buy at $50.00 will not execute at $50.05; a limit order to sell at $100.00 will not execute at $99.95. Limit orders are passive: they rest on the order book and wait for a matching order to arrive. They offer price protection but risk non-execution if the price never reaches the limit.
See also: Market Order, Order Types.
Lit Market
A public exchange or trading venue where orders and prices are displayed transparently in real-time.
The New York Stock Exchange, Nasdaq, and smaller lit venues like CBOE display all bids and asks (level 1 and level 2 data) publicly. Lit markets are the "original" stock exchange model and remain the primary price discovery mechanism. However, lit markets are now competing with dark pools and other ATSs for volume.
Locked Market
A condition where the bid price on one exchange equals the ask price on another, with no spread between them.
A locked market (e.g., Nasdaq bid of $50.00 vs. NYSE ask of $50.00) creates an arbitrage opportunity: buy on one and sell on the other simultaneously for a risk-free profit. Locked markets are rare and fleeting because arbitrageurs exploit them within milliseconds. Regulators monitor locked markets as a sign of potential volatility or trading halts.
LULD
Limit Up-Limit Down — a circuit breaker rule that halts trading in an individual stock when its price moves sharply (up or down 5–20%, depending on volatility) within a very short period.
If a stock normally trades in a $40–60 range and suddenly hits $70 (up 16% in one minute), LULD halts trading for 5 minutes, giving the market time to assess the move. LULD was introduced in 2013 to prevent erratic price movements caused by algorithmic errors or flash crashes. It is more granular than exchange-level circuit breakers.
See also: Circuit Breaker, Volatility.
Maintenance Margin
The minimum equity level (as a percentage of account value) that a margin account must maintain, typically 25% for stocks.
A trader with a $100,000 account, borrowing $50,000 from their broker, must maintain $25,000 in equity (25% of the $100,000 total value). If the market drops and the equity falls below $25,000, the broker issues a margin call demanding the trader deposit additional cash or sell positions. Maintenance margin rules prevent overleveraged positions from triggering large losses for brokers during market dislocations.
See also: Margin Call, Reg T.
Margin Call
A demand from a broker for a margin account holder to deposit additional cash or securities because the account equity has fallen below the maintenance margin requirement.
A trader with a $100,000 stock position financed with $50,000 borrowed from their broker is leveraged 2:1. If the stock drops 30%, the account equity falls to $70,000, but the maintenance margin requirement is $50,000 (25% of $200,000 in gross assets). The account is still above the maintenance threshold — no call is issued. However, if the stock drops 35%, equity falls to $65,000 and the account is still okay. If it drops 40%, equity is $60,000 and the broker issues a margin call for $10,000 (bringing equity back to the required minimum). Failure to meet a margin call within the specified time (usually 2–3 days) results in forced liquidation of the position.
See also: Maintenance Margin, Leverage.
Market Maker
A registered trader or firm that stands ready to buy and sell securities continuously, providing liquidity and earning profit on the bid-ask spread.
Virtu Financial, Citadel Securities, and bank trading desks are major market makers in US equities. They are required to maintain two-sided quotes (bid and ask) within regulatory spread limits, even during volatile periods. In return, they receive regulatory privileges such as exemptions from short-sale restrictions and may receive rebates from exchanges for providing liquidity. Market makers are essential to market function but are controversial when they also operate dark pools or profit from PFOF arrangements.
See also: Bid-Ask Spread, PFOF.
Matching Engine
The software system at the heart of an exchange or ATS that matches buy and sell orders according to a predefined set of rules (price-time priority, pro-rata allocation, etc.).
The Nasdaq matching engine receives a limit order to buy 1,000 shares at $50.10 and simultaneously receives a limit order to sell 1,000 shares at $50.05. The engine matches them at $50.05 (the price of the resting order), executing the trade instantly. Matching engines operate in microseconds and must be fault-tolerant and highly available.
See also: Order Book, Exchange.
NBBO
National Best Bid-Offer — the highest bid and lowest offer across all US equity exchanges and ATSs combined, used as the regulatory benchmark for fair pricing.
If the NBBO is 50.25 bid, 50.26 ask, a broker must not execute a customer's buy order at 50.30, nor a sell order at 50.20, without justification. The SEC uses NBBO to police insider trading, front-running, and trade-through violations. Real-time NBBO data is disseminated to all market participants via the consolidated tape and is fundamental to regulatory oversight.
See also: BBO, Best Execution.
NSCC
National Securities Clearing Corporation — the subsidiary of DTCC that clears and settles most US equity trades, managing counterparty risk and fail resolution.
When you buy 100 shares of Apple from another investor, NSCC stands in the middle: it becomes the buyer to the seller and the seller to the buyer. NSCC guarantees both sides' settlement, nets offsetting trades to reduce cash and share movements, and manages failed trades. NSCC's ability to scale this operation to trillions of dollars daily is a core strength of the US market infrastructure.
See also: Clearinghouse, DTCC, Settlement.
Order Book
The electronic ledger maintained by an exchange or ATS that lists all resting buy and sell orders at each price level, ranked by time priority.
The Apple order book might show:
- Bids: 10,000 shares at $150.50, 8,000 at $150.49, 12,000 at $150.48
- Asks: 7,000 shares at $150.51, 9,000 at $150.52, 15,000 at $150.53
A new market order to buy 10,000 shares executes instantly against the top ask (7,000 shares at $150.51), then consumes 3,000 shares from the second level (at $150.52). The order book is refreshed in real-time on lit exchanges.
See also: Matching Engine, Depth of Market.
PDT
Pattern Day Trader — the SEC and FINRA rule requiring traders who execute four or more round-trip trades in a rolling five-day period to maintain a minimum $25,000 account balance.
A retail investor with a $10,000 account is blocked from day trading under the PDT rule. If they buy and sell Apple twice in one day (two round trips), they are flagged as a day trader and must either cease day trading or deposit $15,000 to reach the $25,000 minimum. The rule is intended to protect undercapitalized traders from catastrophic losses but is also criticized for limiting retail market access.
See also: Day Trader, Margin.
PFOF
Payment for Order Flow — compensation paid by market makers (or ATSs/dark pools) to brokers for routing customer orders to them.
A retail broker receives $0.001 per share from Citadel Securities for routing its retail customer's 1,000-share order to Citadel's dark pool instead of the lit market (say, Nasdaq). If the customer's order would have received the NBBO price on Nasdaq anyway, this is transparent; if the broker routes to Citadel despite a better price being available elsewhere, the broker may be violating best execution. The SEC and regulators have scrutinized PFOF heavily, especially in the meme stock and retail-focused brokerage context.
See also: Best Execution, Dark Pool, Market Maker.
PIPE
Private Investment in Public Equity — a transaction where a private investor or institution buys newly-issued shares of a public company at a negotiated price, typically at a discount to market price.
A company trading at $50 per share conducts a PIPE offering at $48 per share to raise capital quickly. The investor gets a discount in exchange for locking in the purchase for a set period. PIPEs are popular for companies needing cash without going through a formal secondary offering or shelf registration.
See also: Follow-On Offering, Shelf Registration.
Quote Stuffing
Rapid submission and cancellation of non-bona-fide orders intended to confuse competitors or create a false appearance of trading interest.
A high-frequency trader submits 50,000 buy orders for a stock at various prices within microseconds, then cancels them all before any execute. The strategy can create a false appearance of strong demand (causing other algos to buy), temporarily moving the price up, and allowing the spoofing trader to profit. Quote stuffing is illegal under the 2010 Dodd-Frank Act and has resulted in SEC and CFTC enforcement actions.
See also: Spoofing, Market Manipulation.
Reg T
Regulation T (Federal Reserve Board) — the rule limiting the amount a broker may lend to a margin customer to 50% of the purchase price of eligible securities.
A trader buying $100,000 of Apple stock can borrow up to $50,000 from their broker. Reg T is the "initial margin" requirement. Once the position is held, the "maintenance margin" (typically 25%) takes over. Reg T applies to brokers and is enforced by the Federal Reserve and FINRA.
See also: Maintenance Margin, Margin.
Reverse Split
A corporate action in which a company reduces the number of outstanding shares by consolidating multiple shares into one, increasing the price per share proportionally.
A company with 100 million shares at $2 per share executes a 10-for-1 reverse split, resulting in 10 million shares at $20 per share. Reverse splits are often used to raise a delisted stock back above the $1 minimum price threshold on major exchanges, or to improve the image of a struggling company (fewer, higher-priced shares "look better" to some investors, though fundamentals are unchanged). Reverse splits are dilutive from a total market-cap perspective only if the split triggers forced selling or delisting risk.
See also: Stock Split.
Rights Offering
A corporate action in which a company offers existing shareholders the right to buy additional shares at a discounted price, usually to raise capital.
A company offers each shareholder the right to buy one new share for every ten owned at $15, when the stock trades at $20. Shareholders can exercise the right, sell the right to another investor, or let it expire. Rights offerings are dilutive but allow existing shareholders to maintain their ownership percentage if they participate fully.
See also: Follow-On Offering, Dilution.
Settlement
The process of transferring securities from seller to buyer and cash from buyer to seller, completing a trade.
The SEC shortened settlement from T+3 (three business days) to T+1 (next business day) in 2024. When you buy shares at 10:00 AM on Monday, you typically own them and must pay by end of day Tuesday (T+1). The DTCC and NSCC coordinate with custodians and brokers to move shares and cash simultaneously, using the Continuous Net Settlement (CNS) system.
See also: T+1, T+2, DTCC, NSCC.
Shelf Registration
An SEC registration statement allowing a company to issue securities in one or more offerings over a period of up to three years, without filing a new registration for each issuance.
A company files an S-3 shelf registration statement authorizing up to $500 million in new equity offerings over three years. It can then issue $100 million in year one, another $200 million in year two, and so on, without regulatory delays. Shelf registrations streamline capital-raising for large, established companies.
See also: Follow-On Offering, IPO.
Short Interest
The total number (or percentage) of shares of a company that are currently sold short and outstanding.
If Company A has 50 million shares outstanding and 5 million of those are short sold (borrowed and sold), the short interest is 5 million shares, or 10% of the float. High short interest (15%+) can indicate investor skepticism or an opportunity for a short squeeze if positive news drives covering. The SEC requires short interest data to be reported semi-monthly.
See also: Short Selling, Short Squeeze.
Short Selling
Borrowing shares from a broker, selling them immediately, and profiting if the price falls (then buying back at a lower price to return the shares).
A trader borrows 1,000 shares of Tesla from their broker, sells them at $800 per share for $800,000, and hopes to buy them back at $700 per share for $700,000, keeping the $100,000 difference. Short selling is legal but requires a locate (confirmation that shares can be borrowed) and a buy-in may be forced if the lender demands the shares back. Short sellers provide price discovery and prevent bubbles, but can be destabilizing during panics.
See also: Short Interest, Short Squeeze, Uptick Rule.
Short Squeeze
A rapid price surge caused by short sellers rushing to buy back shares when a heavily-shorted stock rises sharply, creating a feedback loop of covering demand.
In January 2021, retail investors on Reddit bought heavily into GameStop, which had 140% of its float short sold. As the stock rose from $20 to $400, short sellers panicked and rushed to cover, buying more shares and driving the price higher, triggering more covering—a classic short squeeze. Eventually the panic subsided, and the price collapsed below $50.
See also: Short Selling, Short Interest.
SIPC
Securities Investor Protection Corporation — a non-profit corporation that protects customer assets held at failing brokers, insuring up to $500,000 per customer per broker.
If your broker declares bankruptcy, SIPC's trustee takes possession of customer assets and distributes cash and securities back to customers. SIPC protection covers cash balances and securities but does not cover losses from bad investments; it protects against broker insolvency and theft. Most major brokers carry SIPC insurance.
See also: Broker.
Spin-Off
A corporate action in which a company distributes shares of a newly-created or separated subsidiary to its existing shareholders as a dividend.
A conglomerate spins off its technology division, issuing one share of the new tech company for every five shares of the parent held. Shareholders now own two separate companies instead of one. Spin-offs are often tax-efficient (relative to a sale) and allow focused management of distinct businesses.
See also: Dividend.
SPAC
Special Purpose Acquisition Company — a shell company created to raise capital via an IPO, with the intent to merge with or acquire an operating company.
A SPAC raises $500 million in an IPO and holds the cash in a trust. The SPAC sponsors identify a private company (often in clean energy, biotech, or fintech) and negotiate a merger, bringing it public without a traditional IPO roadshow. SPACs were popular in 2020–2021 but have fallen out of favor due to poor post-merger returns and regulatory scrutiny.
Spread
The difference between the bid and ask price, a key measure of market liquidity and trading cost.
A $0.01 spread on a $150 stock is tight (0.0067% cost); a $0.50 spread is wide (0.33% cost). Brokers, market makers, and public quotes display spread information as a proxy for liquidity. Tighter spreads benefit all market participants by reducing transaction costs.
See also: Bid-Ask Spread, Liquidity.
SSR
Short Sale Price Test Rule (Uptick Rule Alternative) — a rule halting naked short sales when a stock falls 10% or more in a single day, resuming the next day.
If Apple drops more than 10% on a single day, SSR restricts short selling for the remainder of that day and the next full trading day. SSR is designed to prevent panicked short selling from accelerating declines. It does not affect covered short sales (where the shares are already borrowed).
See also: Short Selling, Uptick Rule.
Stop-Loss
An order that automatically becomes a market order to sell when the price falls to or below a specified level, intended to limit losses.
A trader buys Apple at $150 and places a stop-loss at $140, intending to exit if the position declines by $10. If Apple gaps down to $138 on negative news, the stop-loss triggers and becomes a market order to sell immediately—likely executing near $138, not $140. Stop-losses do not guarantee execution at the stop price and can lock in worse losses during gaps.
See also: Stop-Limit, Order Types.
Stop-Limit
An order combining a stop price trigger with a limit price, executing as a limit order only if and when the stock trades at the stop price.
A stop-limit order to sell 100 shares at a stop price of $140 with a limit price of $139 means: if Apple falls to $140 or below, submit a limit sell order at $139. The order will not execute above $139, protecting the downside. However, if Apple gaps down to $130 without ever trading at $140, the order is never triggered. Stop-limits reduce execution risk but add complexity.
See also: Stop-Loss, Limit Order.
T+1
Trade plus one business day — the current settlement standard in US equities as of 2024, requiring trades to settle by the next business day.
A trade executed Monday settles Wednesday (assuming no weekend in between). The SEC shortened settlement from T+3 to T+1 in 2024 to reduce operational risk and align with international standards. T+1 reduces the window for fails and counterparty default but increases operational pressure on brokers and clearinghouses.
See also: Settlement, T+2.
T+2
Trade plus two business days — the former US equity settlement standard, replaced by T+1 in 2024.
Historically, a Monday trade settled on Wednesday. The SEC moved to T+1 to reduce fails and systemic risk. Most major market participants now settle on a T+1 basis, though some instruments (municipal bonds, foreign currency) may still settle T+2 or later.
See also: T+1, Settlement.
Tender Offer
A public invitation for shareholders to sell (or sometimes buy) shares at a specified price, typically during a takeover or corporate restructuring.
An acquirer makes a tender offer to buy all shares of a target company at $55 per share (a 20% premium to market price). Shareholders can accept the offer and sell their shares, or reject it and keep their shares. The acquirer needs a minimum percentage (often 50%+1) to succeed.
See also: Merger, Acquisition.
Tick Size
The minimum price increment permitted for a stock, currently $0.01 (one cent) for most US equities.
Prices must move in increments of at least $0.01; a stock cannot trade at $50.001 or $50.005 under current rules. Smaller tick sizes (e.g., $0.001) would benefit high-frequency traders by allowing them to improve bids and offers by tiny amounts, cutting into market maker spreads. Regulators have resisted tick-size reductions to protect market maker viability.
See also: Bid-Ask Spread.
Time-in-Force
The duration rule governing how long an order remains active (day, GTC, IOC, FOK, etc.).
Common time-in-force options include Day (expires at market close), GTC (good until cancelled, typically expires after 30–90 days), IOC (immediate-or-cancel), and AON (all-or-none). The choice affects execution timing and the risk of unexpected fills or non-fills.
See also: Day Order, GTC, IOC, FOK.
Trailing Stop
A stop-loss order that automatically adjusts upward as the stock price rises, "trailing" the price by a fixed amount or percentage.
A trader buys Apple at $150 and sets a trailing stop at $5 below the market price. If Apple rises to $160, the stop automatically adjusts to $155 (maintaining a $5 buffer). If Apple then falls to $155 or below, the stop triggers and the position is sold. Trailing stops lock in gains while allowing further upside.
See also: Stop-Loss.
Uptick Rule
A rule (currently in effect) restricting short sales in a stock to upticks (a price equal to or above the previous trade) when the stock is in downtick mode.
If the last trade was at $50 and the previous trade was at $49.99 (a downtick), short sellers cannot sell at $49.99; they must wait for an uptick to $50 or higher. The uptick rule is designed to prevent short sellers from accelerating declines but is controversial because it can interfere with normal trading and may be ineffective at preventing crashes.
See also: Short Selling, SSR.
VWAP
Volume-Weighted Average Price — the average price of a security weighted by trading volume at each price level, used as a benchmark for execution quality.
If 1,000 shares trade at $100 and 2,000 shares trade at $101, the VWAP is $100.67 (2,000 shares $101 + 1,000 shares at $100) / 3,000 shares). Institutional traders often target VWAP execution to ensure they do not overpay relative to the day's actual trading pattern. Algorithms like "VWAP algorithms" break large orders into smaller pieces and execute them throughout the day to hit the VWAP.
See also: Execution.