Glossary
What is an order book?
Also called: orderbook, depth, market depth, DOM
Updated Ninja Terminal team
Definition
An order book is a real-time list of all open buy orders (bids) and sell orders (asks) for a market, sorted by price. Each price level shows the total size resting there. Matching engines pair incoming orders against the book, so the book shows available liquidity and the likely price of a trade.
Every limit order that is not yet filled rests in the book. Bids sit below the current price and asks sit above it. The top level is the best bid and offer.
How does matching work?
A new order is matched against the opposite side, starting at the best price. Most exchanges use price-time priority: better prices fill first, and at the same price the older order fills first.
- A market order removes liquidity and walks through levels until it is filled.
- A limit order priced at or through the opposite best price fills at once. If it is priced behind that, it rests and adds liquidity.
Example: asks show 1 BTC at 60,005, 2 BTC at 60,010, and 3 BTC at 60,020. A market buy for 2.5 BTC fills 1 at 60,005, 1.5 at 60,010 and moves the best ask to 60,010 (with 0.5 BTC left).
What can you read from depth?
- Spread: the gap between best bid and ask. See bid-ask spread.
- Depth: how much size sits within a given distance of the price. Deep books mean less slippage.
- Imbalance: more size on one side may hint at pressure, but resting orders can be canceled at any time, so treat it with care.
Each exchange keeps its own book. The same coin can show a different spread and depth on Binance, OKX, Bybit, and Hyperliquid.
Where does it appear in Ninja Terminal?
Ninja Terminal has an order book widget with the live spread for the selected market. The Quick Entry preview also shows the estimated slippage for your size.
Frequently asked questions
What is the difference between bids and asks?
Bids are buy orders and asks (offers) are sell orders. The best bid is the highest buy price. The best ask is the lowest sell price.
Can the order book be misleading?
Yes. Resting orders can be canceled before they fill, and large orders can be hidden or split. Use depth as context, not as a forecast.