Order Book

The live list of all resting buy and sell orders at each price level.

Also called: depth of market · dom · level 2

In plain language

The order book shows unfilled limit orders stacked by price: bids below the current market, asks above it. The top of each side is the best bid and best ask.

Depth is the quantity waiting at each level. A book with thousands of shares at every price absorbs large orders quietly; a book with a handful of shares gaps.

The book is a snapshot of intent, not a promise. Resting orders can be pulled in an instant, and often are, exactly when a large order starts pushing into them.

Seen on a chart

Asks rest above the market, bids below. The best of each pair is the top of the book, and the gap between them is the spread.

Why it matters

Reading the book tells you what your fill will realistically look like. Position size math assumes you can transact at your entry — the book is where you check that assumption.