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.

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.

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%.

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.

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.