Quick answer
Order book: An order book is the list of open buy (bid) and sell (ask) orders for a contract, matched by price and time; a central limit order book (CLOB) is the electronic version most exchanges use.
Also called: central limit order book
Explained, with an example
The CFTC defines an order book as a market structure where bids and offers are matched based on price and/or arrival time [1]. On Polymarket, the displayed price is the midpoint of the best bid and ask — e.g. bids at $0.34 and asks at $0.40 show $0.37 [2]. The gap between the two is the Bid-ask spread, and the volume resting near the top of the book is a practical measure of Liquidity.
Formula
midpoint = (best bid + best ask) / 2Try it: tools and tutorials
Frequently asked questions
What is an order book?
An order book is the list of open buy (bid) and sell (ask) orders for a contract, matched by price and time; a central limit order book (CLOB) is the electronic version most exchanges use.
About this guide
Pred-Data publishes independent, source-linked explainers about prediction markets and the AI tools used around them. Every factual claim on this page links to a primary or reputable secondary source, checked on the date shown above. We do not run a trading venue, take bets, or give personal advice.
Author: Ken Lin · Corrections: if you spot an error, we review it and date-stamp the fix.
Affiliate disclosure
Some links on this site (marked “Partner link”) may be affiliate links: if you sign up through them we may earn a commission at no extra cost to you. Partner links use rel="sponsored nofollow" and never change our assessment. Platform links in our comparison tables are plain, non-affiliate links to official sites.
Sources
- CFTC Glossary (binary option, DCM, order book, liquid market, market maker, settlement, wash trading, arbitrage, hedging) — U.S. CFTC, accessed 2 Nov 2026
- Prices & Orderbook — Polymarket Documentation, accessed 2 Nov 2026