Glossary
Order book
A real-time list of all outstanding buy and sell orders for an asset, ranked by price.
Also called: limit order book, depth of market
An order book is the live record of all outstanding buy orders, or bids, and sell orders, or asks, for a given asset on an exchange or trading venue, organized by price level. It shows not just the current best price but the full depth of interest behind it — how much is waiting to trade at each price.
The highest bid and lowest ask together form the top of the book, and the gap between them is the bid-ask spread. Orders below the top of book, at less attractive prices, make up the book's "depth"; a book with large size stacked near the top price is considered liquid, while a thin book can move significantly on a modest order. This differs from a simple quote or last-traded price, which shows only where the market last transacted, not what is waiting to trade next.
Traders and algorithmic trading systems read the order book to gauge short-term supply and demand and to time entries and exits, a practice sometimes folded into technical analysis as order-flow reading. A common pitfall is treating visible order book size as a firm signal of intent: orders can be placed and cancelled within milliseconds, and some participants intentionally post and pull orders to influence perceptions of depth, a practice regulators in several jurisdictions have restricted.
Last reviewed September 22, 2026