# Trading and Markets Glossary

> 101 terms from the Investing & Digital Assets book's Trading and Markets part, each defined in a line and linked to the chapter that explains it.

- **0DTE (zero days to expiration):** an option bought or sold on its expiration day; a majority of US index option volume by 2026 ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#expiration-day-and-0dte)).
- **23/5 trading:** exchange trading of NMS stocks 23 hours a day, five days a week, with an overnight session from 8:00 pm to 4:00 am ET and a one-hour pause from 8:00 to 9:00 pm Monday to Thursday, planned from December 6, 2026 ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#23-5-on-exchanges)).
- **30.7 secured amount:** the separate pool an FCM holds for customers trading on foreign futures exchanges ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#customer-segregation)).
- **Access fee cap:** the most an exchange may charge to execute against its protected quotations under Rule 610(c): $0.003 a share today, $0.001 a share (0.1% of the quote below $1.00) once the 2024 amendments take effect in November 2027 ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#access-fees-and-tick-sizes)).
- **After-hours session:** exchange and ATS trading from 4:00 pm to 8:00 pm ET, limit orders only on most venues ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#the-sessions-today)).
- **Alternative trading system (ATS):** a trading system operated by a broker-dealer under Regulation ATS rather than registered as an exchange; an NMS Stock ATS needs an effective Form ATS-N ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#the-venues)).
- **American-style option:** an option that can be exercised on any business day up to expiration, so a short position can be assigned early ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#contracts-series-and-symbols)).
- **Assignment:** OCC's notice that an exercise has been matched to a short position, allocated by the clearing firm to a customer first in first out or at random ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#exercise-and-assignment)).
- **Best execution:** a broker's duty under FINRA Rule 5310 to use reasonable diligence to find the best market for a customer's order so the price is as favorable as possible under prevailing conditions ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#the-duty)).
- **Best odd-lot order:** the highest-priced odd-lot bid above the national best bid, or lowest-priced odd-lot offer below the national best offer, added to odd-lot information by the 2024 amendments ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#the-nbbo-worked)).
- **Book checksum:** a hash of a venue's top book levels sent with its feed (Kraken's CRC32 of the top 10 levels) so a client can prove its rebuilt book matches the venue's ([Market Data](https://www.syntaqx.com/books/investing-and-digital-assets/market-data#crypto-websocket-books)).
- **Borrow fee:** the annualized rate a short seller pays to borrow shares, charged daily on the short's value (Alpaca and Robinhood divide by 360) ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#short-selling-and-regulation-sho)).
- **Cancel/replace race:** a cancel or replace request that crosses a fill in flight, so the venue applies it to a partly or fully filled order; FIX replaces state the total quantity to avoid doubling a position ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#cancel-replace-races)).
- **Candle:** the open, high, low, close and volume of trades in an interval, identified by its open time and recomputable when late, busted or corrected trades change it ([Market Data](https://www.syntaqx.com/books/investing-and-digital-assets/market-data#candles-and-aggregation)).
- **ClOrdID chain:** the FIX client order IDs linking an order to its replacements; each replace carries a new ClOrdID and OrigClOrdID naming the previous non-rejected order ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#fix-basics)).
- **Closing auction:** the single-price auction that sets a listed stock's official close, with entry and cancel cutoffs (3:50 p.m. ET at NYSE) and imbalance messages before it ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#auctions)).
- **Collar (market order):** a broker's conversion of a market order into a limit order some distance from the quote so it can't fill through a thin book; a material term the customer must be told ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#order-types)).
- **Conflation:** sending each client only the latest state per symbol at the client's rate rather than every update, correct for quotes and charts, wrong for consumers that need every event ([Market Data](https://www.syntaqx.com/books/investing-and-digital-assets/market-data#fan-out-to-millions-of-clients)).
- **Contrary Exercise Advice:** the form a member submits by 7:30 p.m. ET on expiration day to exercise or not exercise a customer's expiring option against the exercise-by-exception default ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#exercise-and-assignment)).
- **Covered call:** a short call against 100 shares held per contract; income now, upside capped at the strike ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#payoffs-and-requirements)).
- **CT Plan:** the single consolidated equity data plan, effective November 20, 2024, that on implementation replaces the CTA, CQ and UTP plans ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#sips-and-direct-feeds)).
- **Day margin:** an FCM's lower margin for a futures position opened and closed within a session, ending at a set time before the close ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#margin-and-daily-settlement)).
- **Derivatives clearing organization (DCO):** a CFTC-registered clearing house that becomes the counterparty to every cleared futures trade, sets margin and settles with clearing members at least once each business day ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#the-parties)).
- **Designated contract market (DCM):** a CFTC-registered futures exchange, which lists contracts by self-certification or approval ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#the-parties)).
- **Direct feed:** an exchange's own market data product, with depth or order-by-order detail, that reaches a subscriber without the hop through the SIP ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#sips-and-direct-feeds)).
- **Early assignment:** assignment of an American-style short option before expiration, most often on a call the day before an ex-dividend date when the dividend exceeds its time value ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#early-assignment-and-dividends)).
- **Effective spread:** twice the distance between a fill price and the NBBO midpoint at order receipt; divided by the quoted spread it gives E/Q, the share of the spread the customer paid ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#one-order-worked)).
- **Event contract:** a derivative that pays a fixed amount, usually $1, if a stated event happens and nothing if it doesn't, priced between one cent and 99 cents ([Event Contracts & Prediction Markets](https://www.syntaqx.com/books/investing-and-digital-assets/prediction-markets#pricing-price-as-probability)).
- **Exercise by exception (Ex-by-Ex):** OCC Rule 805's automatic exercise of expiring equity options in the money by a specified amount unless instructed otherwise; brokers publish $0.01 ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#exercise-and-assignment)).
- **Futures commission merchant (FCM):** a firm registered with the CFTC and a member of NFA that takes customers' futures orders and money and must segregate customer funds ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#the-parties)).
- **Hard to borrow:** a security whose shares are scarce in the lending market, needing an approved locate before a short sale and carrying a borrow fee ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#hard-to-borrow-and-securities-lending)).
- **House requirement:** a firm's own margin requirement above the Reg T and FINRA minimums, set by security and changeable without notice ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#maintenance-and-house-requirements)).
- **IML (intraday margin level):** under FINRA Rule 4210, the cash a margin account could withdraw while still meeting maintenance margin, negative when it is below ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#intraday-margin-after-the-pattern-day-trader-rule)).
- **Initial margin (futures):** the collateral a new futures position requires, set by the clearing house and often raised by the FCM ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#margin-and-daily-settlement)).
- **Initial margin (securities):** the equity required to open a position: 50% of market value for a margin equity security and 150% for a short sale under Regulation T ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#reg-t-initial-margin)).
- **Intermarket sweep order (ISO):** a limit order marked as one of a set sent at once to take the full displayed size of every better protected quotation, letting the receiving venue execute without checking for trade-throughs ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#order-protection)).
- **Intraday margin deficit:** for a day with an IML-reducing transaction, at least the largest negative IML that day; satisfied by later net deposits or IML increases, outstanding for 15 business days ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#intraday-margin-after-the-pattern-day-trader-rule)).
- **Introducing broker (futures):** a futures firm that takes orders but holds no customer money and clears through an FCM, with $45,000 of capital or an FCM's guarantee ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#the-parties)).
- **Kill switch:** a pre-built action that stops order flow and cancels resting orders at a scope (session, account, strategy, venue connection or firm) ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#kill-switches)).
- **L1, L2, L3 data:** top of book (best bid and offer, last trade), depth aggregated by price level, and every resting order with every event on it ([Market Data](https://www.syntaqx.com/books/investing-and-digital-assets/market-data#l1-l2-and-l3)).
- **Limit state:** under LULD, a stock whose best offer is at the lower band or best bid at the upper; a pause follows if it doesn't exit within 15 seconds ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#limit-up-limit-down)).
- **Limit Up-Limit Down (LULD):** the NMS plan that keeps each stock's trades within percentage bands around a five-minute average price and pauses it for five minutes when it can't ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#limit-up-limit-down)).
- **Locate:** under Regulation SHO Rule 203(b), the documented borrow, arrangement or reasonable grounds a broker must have before accepting a short sale order ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#short-selling-and-regulation-sho)).
- **Maintenance margin (futures):** the equity floor below which a futures account is called ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#margin-and-daily-settlement)).
- **Maintenance margin (securities):** the equity a margin account must keep: at least 25% of long market value under FINRA Rule 4210, more for shorts and under house rules ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#maintenance-and-house-requirements)).
- **Margin call:** a demand for deposit when an account breaks a Reg T, maintenance or house requirement; the firm may sell instead of waiting ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#margin-calls-and-liquidations)).
- **Margin level:** a crypto venue's ratio of equity to used margin (Kraken) or collateral to liabilities (Binance), with call and liquidation thresholds ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#crypto-margin-and-borrowing)).
- **Market access rule (Rule 15c3-5):** the SEC rule requiring brokers with market access to keep pre-trade credit, capital, erroneous-order and regulatory controls under their direct and exclusive control ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#the-market-access-rule)).
- **Market-wide circuit breaker:** a halt of all US equity trading after the S&P 500 falls 7% or 13% (15 minutes, before 3:25 p.m.) or 20% (the rest of the day) from the prior close ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#market-wide-circuit-breakers)).
- **Marketable limit order:** a buy limit at or above the national best offer (or a sell at or below the national best bid) when received, which can trade at once and is treated like a market order in routing and reporting ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#order-types)).
- **Mention market:** an event contract on whether a speaker says a given word, which the CFTC's staff warned presents a heightened risk of manipulation ([Event Contracts & Prediction Markets](https://www.syntaqx.com/books/investing-and-digital-assets/prediction-markets#settlement-and-resolution)).
- **Minimum pricing increment (tick size):** the smallest price step for quotes and orders under Rule 612: $0.01 for stocks at $1.00 or more, and $0.005 for stocks with a time-weighted average quoted spread of $0.015 or less once the 2024 amendments take effect ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#access-fees-and-tick-sizes)).
- **National best bid and offer (NBBO):** the best bid and best offer for an NMS stock across all exchanges and FINRA, calculated and disseminated under an NMS plan, ties ranked by size then time ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#the-nbbo-worked)).
- **NFA:** the National Futures Association, the futures industry's self-regulatory organization, of which every registered FCM is a member ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#the-parties)).
- **Non-display use:** market data consumed by machines (routing, risk, matching, derived calculations) rather than shown to people, licensed by category ([Market Data](https://www.syntaqx.com/books/investing-and-digital-assets/market-data#entitlements-delayed-data-and-display-rules)).
- **Not held order:** an order where the customer gives the broker discretion over price and time; Rule 606's quarterly statistics exclude them, and brokers often treat fractional and dollar-based orders this way ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#extended-hours-and-notional-orders)).
- **Options Disclosure Document (ODD):** "Characteristics and Risks of Standardized Options," which Rule 9b-1 requires a broker to deliver before approving an account for options or accepting an order ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#approval-and-the-disclosure-document)).
- **Order management system (OMS):** the system that accepts orders, checks them against the account, sends them to venues, applies fills and cancels, and keeps each order's state and history ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#the-oms-lifecycle-and-state-machine)).
- **Order protection rule (Rule 611):** the requirement that trading centers prevent trade-throughs of protected quotations, with listed exceptions; its rescission was proposed in June 2026 ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#order-protection)).
- **Overnight ATS:** an alternative trading system that matches NMS stocks between 8:00 pm and 4:00 am ET, such as Blue Ocean, Bruce, IBKR Eos and MOON ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#overnight-atss-and-the-2024-halt)).
- **Overnight LULD bands:** static price bands for the overnight session, 20% around the official close and the evening last round-lot sale (7:45 pm, moving to 7:30 pm under Amendment 29), with minimum widths of $1.00 or $3.00 and no automatic pause ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#what-had-to-change-around-the-exchanges)).
- **Part 40.11:** the CFTC rule barring event contracts that involve terrorism, assassination, war, gaming or unlawful activity, with a 90-day review ([Event Contracts & Prediction Markets](https://www.syntaqx.com/books/investing-and-digital-assets/prediction-markets#the-rules)).
- **Pattern day trader:** the former FINRA designation for four or more day trades in five business days, with a $25,000 minimum; replaced by the intraday margin standard from June 4, 2026 ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#intraday-margin-after-the-pattern-day-trader-rule)).
- **Portfolio margin:** a risk-based margin method under FINRA Rule 4210(g) that sets the requirement from the worst loss across price scenarios ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#portfolio-margin)).
- **Position closing only:** a firm's restriction that lets customers sell or close positions in a security but not open or add to them, as brokers imposed in January 2021 ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#clearing-deposits-and-the-january-2021-restrictions)).
- **Pre-market session:** exchange and ATS trading from 4:00 am to 9:30 am ET ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#the-sessions-today)).
- **Prediction market:** a market in event contracts, in the US a CFTC-registered DCM and DCO reached through an FCM ([Event Contracts & Prediction Markets](https://www.syntaqx.com/books/investing-and-digital-assets/prediction-markets#how-the-platforms-do-it)).
- **Price collar:** a pre-trade check rejecting a limit price too far through the current market, the control that catches a mistyped price ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#checks-before-an-order-leaves)).
- **Price improvement:** the amount by which a fill beats the NBBO at receipt (below the offer for a buy, above the bid for a sell), per share and in total; a worse price is disimprovement ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#one-order-worked)).
- **Price-time priority:** the matching rule in which the best price trades first and, at one price, the earliest order; trades happen at the resting order's price ([Order Books & Matching Engines](https://www.syntaqx.com/books/investing-and-digital-assets/order-books-and-matching#price-time)).
- **Pro-rata allocation:** a matching rule that splits fills at a price level in proportion to resting size rather than by arrival time, used in some futures and options markets ([Order Books & Matching Engines](https://www.syntaqx.com/books/investing-and-digital-assets/order-books-and-matching#pro-rata)).
- **Professional subscriber:** a market data user whose use is by or for an entity or serves third parties for compensation, licensed at professional rates; anyone else is non-professional ([Market Data](https://www.syntaqx.com/books/investing-and-digital-assets/market-data#entitlements-delayed-data-and-display-rules)).
- **Protected quotation:** an exchange's or FINRA's best bid or offer, displayed by an automated trading center as a round lot and disseminated under an NMS plan, which other venues may not trade through ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#the-nbbo-worked)).
- **Realized spread:** the effective spread measured against the midpoint some time after execution (15 seconds to 5 minutes in Rule 605), showing how much the filler kept once prices moved ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#reading-a-rule-605-report)).
- **Regular and rigorous review:** the at-least-quarterly review, by security and order type, of execution quality against competing markets that FINRA 5310 requires of firms that don't review order by order ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#the-duty)).
- **Regulation SHO:** the SEC's short sale rules: order marking, the locate, the close-out of fails, threshold securities and the Rule 201 price test ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#short-selling-and-regulation-sho)).
- **Regulation T:** the Federal Reserve's rule (12 CFR 220) on broker credit: initial margin, cash account payment and calls within one payment period ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#reg-t-initial-margin)).
- **Residual interest:** an FCM's own money in a customer segregated account, which must cover undermargined customers by 6:00 pm ET on the settlement date ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#customer-segregation)).
- **Round lot:** the order size whose quotes count for the NBBO and protection, set by a stock's average closing price: 100 shares at $250 or less, 40 to $1,000, 10 to $10,000 and 1 above ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#round-lots-and-odd-lots)).
- **Rule 201 (short sale price test):** after a 10% decline from the prior close, short sales may not be priced at or below the national best bid for the rest of the day and the next ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#trading-halts)).
- **Rule 2265 disclosure:** FINRA's required extended-hours risk disclosure naming lower liquidity, higher volatility, changing prices, unlinked markets, news announcements and wider spreads ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#the-sessions-today)).
- **Rule 605 report:** a monthly public report of execution quality statistics by order type and size by market centers and larger brokers, amended in 2024 with compliance from August 1, 2026 ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#reading-a-rule-605-report)).
- **Rule 606 report:** a broker's quarterly public report of where its non-directed orders went, by month and order type, with payments received and the material terms of each venue relationship ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#reading-a-rule-606-report)).
- **Securities information processor (SIP):** the processor under an equity data plan that consolidates exchanges' quotes and trades and publishes the NBBO and the consolidated tape ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#sips-and-direct-feeds)).
- **Segregation computation:** the FCM's daily statement, by currency, of customer funds held, the amount required and its residual interest, filed by noon the next business day ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#customer-segregation)).
- **Self-certification:** listing a new contract by filing it and certifying it complies with the Act by the open of business the business day before listing (17 CFR 40.2) ([Event Contracts & Prediction Markets](https://www.syntaqx.com/books/investing-and-digital-assets/prediction-markets#listing)).
- **Self-trade prevention:** matching engine rules that stop an account's order from trading with its own resting order, by canceling or decrementing one or both under the mode chosen ([Order Books & Matching Engines](https://www.syntaqx.com/books/investing-and-digital-assets/order-books-and-matching#self-trade-prevention)).
- **Sequencer (matching engine):** the component that assigns every input to a matching engine its place in one order and makes the log durable, so the book is a deterministic function of the log ([Order Books & Matching Engines](https://www.syntaqx.com/books/investing-and-digital-assets/order-books-and-matching#determinism-and-sequencers)).
- **Series (option):** one option type, strike and expiration on one underlying, within the class of all options on it ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#contracts-series-and-symbols)).
- **Smart order router:** software that splits and sends an order across venues by price, fees, fill probability and order protection, recording each decision for best execution review ([Order Routing, PFOF & Best Execution](https://www.syntaqx.com/books/investing-and-digital-assets/routing-and-best-execution#smart-order-routers)).
- **Snapshot and replay:** recovering a matching engine by restoring its state at a sequence number and reapplying every later input; snapshot frequency sets recovery time ([Order Books & Matching Engines](https://www.syntaqx.com/books/investing-and-digital-assets/order-books-and-matching#replay-and-recovery)).
- **Source agency:** the named source an event contract's rules use to determine the outcome ([Event Contracts & Prediction Markets](https://www.syntaqx.com/books/investing-and-digital-assets/prediction-markets#settlement-and-resolution)).
- **Stop and stop limit orders:** orders dormant until the market reaches a stop price, then sent as a market order or a limit order; what "reaches" means (a trade or a quote) is each platform's rule ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#order-types)).
- **Threshold security:** a security with fails of 10,000 shares or more and 0.5% of shares outstanding for five settlement days, on an SRO list; fails in it must be closed out after 13 days ([Margin & Short Selling](https://www.syntaqx.com/books/investing-and-digital-assets/margin-and-short-selling#short-selling-and-regulation-sho)).
- **Time in force:** how long an order stays live: day, good till canceled, good till date, immediate or cancel, fill or kill, or for the opening or closing auction only ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#time-in-force)).
- **Trade date roll:** the 8:00 pm ET boundary after which an execution belongs to the next trade date for CAT, trade reporting and settlement, from December 6, 2026 ([Extended Hours & 24-Hour Trading](https://www.syntaqx.com/books/investing-and-digital-assets/extended-and-overnight-trading#what-had-to-change-around-the-exchanges)).
- **Trading pause:** the five-minute LULD halt in a single stock after a limit state lasts 15 seconds, ended by a reopening auction ([Pre-Trade Risk, Halts & Volatility](https://www.syntaqx.com/books/investing-and-digital-assets/pre-trade-risk-and-halts#limit-up-limit-down)).
- **Trailing stop:** a stop whose price follows the market by a fixed amount or percentage from its best level since entry, which the platform must persist ([Orders & Order Types](https://www.syntaqx.com/books/investing-and-digital-assets/orders-and-order-types#order-types)).
- **Variation margin:** the day's gain or loss on a futures position, paid in cash from the losers to the winners through the clearing house at each daily settlement ([Futures & FCMs](https://www.syntaqx.com/books/investing-and-digital-assets/futures-and-fcms#margin-and-daily-settlement)).
- **Vertical spread:** a long and a short option of the same type and expiration at different strikes; its requirement equals its maximum loss ([Options](https://www.syntaqx.com/books/investing-and-digital-assets/options-trading#payoffs-and-requirements)).
- **Wholesaler:** a broker-dealer that fills retail orders as principal off-exchange, usually at or inside the NBBO, and pays brokers for the flow ([US Equity Market Structure](https://www.syntaqx.com/books/investing-and-digital-assets/equity-market-structure#the-venues)).

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