Order Book

An order book is a real-time record of all outstanding buy and sell orders for a specific cryptocurrency on a trading venue, usually a centralized exchange or an on-chain exchange. It displays bids (buy orders) and asks (sell orders) at different price levels, and it continuously updates as traders place, cancel, or fill orders.

At its core, the order book is the mechanism that enables matching and price discovery. When a trader submits a limit order, for example, “buy 1 ETH at a set price,” that order sits in the book until a seller is willing to trade at that price. When a trader submits a market order, it is executed immediately against the best available prices already listed, moving through multiple price levels if the order is large. The highest bid and lowest ask form the spread, which is often used as a quick indicator of trading costs and market competitiveness.

Because the order book groups orders by price, it also reveals market depth, meaning how much buying or selling interest exists at each level. A “thick” order book with large orders close to the current price typically signals higher liquidity and smaller slippage. A “thin” order book can lead to sharp price moves when market orders consume available liquidity. For instance, in a low-liquidity token pair, a single market buy might clear several ask levels and push the execution price higher than expected.