The words, defined once

Most of the confusion in this sector is vocabulary rather than mechanism. Two venues publish “3%” and mean different amounts; two platforms publish a Brier score under conventions that differ by a factor of two. Each entry below says what the word means here, and the ones that need more than a definition have a page of their own.

Automated market makeralso AMM: A funded pricing formula that stands in for a counterparty. It always has a quote, and the quote moves against you as your order grows, which is how it stays solvent. Liquidity providers put up the money behind it and hold whatever position it took. The trap is that a curve shows a confident two-decimal probability on a market nobody has traded, because a curve cannot be empty.
Brier scorealso quadratic score: The squared distance between a stated probability and what happened, averaged over forecasts, where lower is better. Brier's 1950 original sums that distance over every answer option, so a yes/no forecast scores between 0 and 2. The common shortcut scores one option and runs from 0 to 1. The same forecast is twice as large on the first scale, and platforms rarely say which one they print.
Calibrationalso reliability diagram, calibration curve: Whether the things forecast at a given probability happen at that rate: of everything priced or forecast at 70%, about 70% should come true. It is a property of a set of forecasts, never of a single one, and it says nothing about whether the forecasts were useful. In this catalogue the word also names a job, a capability flag and a correction applied to a model's outputs, and the four uses are not interchangeable.
Central limit order bookalso CLOB, order book: A matching engine that holds every resting bid and ask for one outcome in one queue, ranked by price and then by time, and pairs them when they cross. The venue never takes the other side and never quotes, so if nobody is resting an order there is no price at all. It is the shape most of this catalogue is built on, and it is not a claim about depth.
Close-onlyalso close-only mode, position closing only, PCO restriction: An account or jurisdiction status under which you may reduce or exit positions you already hold but may not open new ones. It is a restriction, not a block: the money is not frozen, but getting it out before resolution needs somebody willing to take the other side. Venues apply it to a country, a state, a detected VPN, an unanswered identity request or an open margin call.
Conditional tokenalso conditional tokens, CTF token, Conditional Tokens Framework: The ERC-1155 token a Polymarket position actually is: one outcome of one question, created by the Gnosis Conditional Tokens contracts against locked collateral. One unit of collateral splits into a complete set, one token per outcome, and a complete set merges back into collateral. Once the oracle reports, each token redeems for its share of the payout. It is a claim on collateral, not a coin with a supply of its own.
Deposit walletalso signature type 3, sigtype 3: On Polymarket, a smart-contract wallet owned by your signing key, deployed for you by the venue's relayer, which holds your collateral and positions and signs orders under signature type 3. Its address is computed from your key before it exists. On a centralised exchange the same words usually mean an address the exchange controls and credits to your account, which is close to the opposite arrangement.
Designated contract marketalso DCM: A US exchange the CFTC has designated under section 5 of the Commodity Exchange Act, and which must keep meeting that section's 23 core principles to stay designated. It is a status of a legal entity, not of a brand, an app or a contract: it names who runs the rulebook and answers to the regulator, not who holds your money, who your broker is, or whether a contract is lawful where you live.
Effective fee at fifty centsalso effective fee at 50 cents, effective taker fee at 50 cents: This catalogue's own unit for a trading fee. Venues publish fees on four incompatible bases, so every card normalises the charge on one contract bought at a price of 50 cents, expressed as a percentage of the dollar a winning contract pays. That is what economics.fees_taker and economics.fees_maker hold. A published 3.00% can be a smaller charge than a published 1.75%.
Event contractalso event contracts, binary event contract: A contract whose payout depends on whether something happens, usually a fixed amount such as one dollar if it does and nothing if it does not, traded at a price between the two. The CFTC uses the phrase descriptively; it is not a defined term in the Commodity Exchange Act. Here it is the neutral noun for any such position, on a regulated exchange or on-chain, and says nothing about who regulates it.
Futures commission merchantalso FCM, futures broker: A firm registered with the CFTC that takes customers' orders for futures, options or swaps and also holds the money that secures them. On a US event exchange it is the broker between you and the designated contract market: it opens your account, runs the identity check, holds your collateral and may add its own fee. The exchange still writes the rules the contract settles by.
Implied probabilityalso market-implied probability, price as probability: The price of a binary contract that pays one dollar, read as the chance that it settles YES: 37 cents reads as 37 percent. It is a reading of a price, not a measured probability. Which price is read, the spread, the fee, the venue's tick grid and its rule text all move the number, so two screens can show different probabilities for what looks like one event.
Maker feealso maker rebate, maker fees: What a venue charges, or pays, the side of a trade whose order was already resting in the book when the other side arrived. Zero is the commonest value on the venue cards here, but it runs from a rebate paid at the fill to a charge equal to the taker's, and on some venues it applies only to named series. It is charged on execution, so a resting order that is cancelled costs nothing.
Material non-public informationalso MNPI, material nonpublic information, material, non-public information: Information a reasonable person would consider important in deciding whether to trade, and which has not been disseminated so that the trading public can generally get it. On event contracts the phrase sets different tests in different documents: Kalshi's rulebook bars trading by anyone who holds it about a contract's underlying, while the CFTC's general rule and Polymarket US reach trading on confidential information only where it breaches a duty owed to the source.
Neg-riskalso negRisk, NegRisk, negative risk, neg risk market, augmented negative risk: Polymarket's structure for an event with several mutually exclusive outcomes, built as a group of ordinary YES/NO markets of which exactly one resolves YES. An adapter contract lets a holder convert NO shares in some outcomes into YES shares in all the others plus cash, which ties the separate order books together. APIs flag membership as negRisk, and orders and conversions go to different contracts.
Optimistic oraclealso UMA oracle: A way of putting an answer on chain without anyone reading it in from a source. Somebody proposes the answer and posts a bond; if nobody disputes it within a fixed window, it becomes final by default. Only a dispute brings in a verifier, which for UMA is a vote of staked token holders. The word describes a default that holds unless challenged, not a data feed.
Parimutuel poolalso parimutuel, pari-mutuel: A market with no counterparty and no quote. Everyone backing an outcome pays into that outcome's pool, the pools close, and the winning pool divides the whole stake after the operator's cut. Your return is fixed only at settlement, so it falls if your side gets crowded after you commit, and there is nothing to sell before then.
Peer scorealso Metaculus Peer score: A score for a forecast measured against the other forecasts on the same question rather than against the outcome alone. On Metaculus, where the name comes from, it is the average gap between your log score and everyone else's, multiplied by 100 and averaged over the life of the question, and all participants' scores on a question add up to zero. Elsewhere the phrase is used loosely for any crowd-relative score, which is where comparisons go wrong.
Proper scoring rulealso proper score, strictly proper: A rule for scoring a probability under which stating the probability you actually hold earns the best expected score, and no other report does better on average. It is strictly proper when nothing else ties. Brier and log scores are both proper. The property belongs to the per-question rule. A platform can rescale it or subtract a crowd without losing it, but a benchmark computed from the same sample, or a nonlinear prize on top, can break it.
Session keyalso session keys, delegated signer: A second signing key that the owner of a Polymarket deposit wallet authorises to trade for it, within named venues and for at most 180 days, and can revoke. The owner's own key is never handed over. Outside this sector the phrase usually means a throwaway key for one connection; here it is a standing delegation that lasts half a year unless somebody ends it.
Settlement sourcealso resolution source, source agency: The named publication, data feed or organisation whose figure decides a contract, as written in its rules before trading opens: a government report, a league's official result, a named news outlet, a price feed. It is not whoever reads that figure and declares the outcome, and it is not the question in the market's title. Two venues listing the same question can name different sources and settle differently.
Slippagealso slippage tolerance, price impact: The gap between the price shown when you decided to trade and the average price your order actually filled at. On a prediction market it is counted in cents on a one-dollar contract, and each cent is a percentage point of probability: pay 53 cents on average for a contract shown at 50 and you need the event to be more than 53 percent likely just to break even, before fees.
Taker feealso taker, taker fees: What a venue charges the side of a trade that removes liquidity, meaning the order that matches immediately against something already resting. The resting side is the maker, and on most venues here the maker pays nothing or is paid. A taker fee is therefore not the cost of trading but the cost of being in a hurry, and it is the only side most schedules publish.