Investing term
What is Order book?
The running list of every bid (offer to buy) and ask (offer to sell) waiting at an exchange, sorted by price.
The order book is the live, sorted list of every resting buy order (bid) and sell order (ask) at an exchange, organised by price. It's the raw supply and demand for a security laid out in real time: bids stacked below the current price, asks above it, each with a size. A trade happens the moment a buy price and a sell price meet.
The depth of the order book is what liquidity really means. A deep book — lots of orders at many closely spaced prices — absorbs large trades with little price impact. A thin book — few orders, big gaps between prices — means even a modest trade can jump the price by sweeping through the sparse levels. Reading the book shows why some securities are cheap to trade and others quietly expensive.
The order book lists every resting bid and ask by price. A deep book absorbs large trades with little price impact; a thin one lets even a modest trade jump the price.
For example
A large market buy eats through several ask levels in a thin order book, filling at progressively higher prices — visible slippage as the book empties.
Learn it by doing
That's Order book in theory — it clicks when you use it. Practise it hands-on in a free, interactive lesson (Stage 5, How Markets Work Globally).
Try the free lesson →Why it matters to you
The order book matters because it's the ground truth behind the concepts investors feel but rarely see: the spread, liquidity, and slippage all live in its shape. A deep book means your trades barely move the price; a thin one means they can. You don't need to watch it tick, but understanding that every price you're quoted comes from a stack of real orders — and that the stack can be shallow — explains why liquidity is worth paying attention to.
⚠ Assuming there's always enough on the other side
A quoted price implies you can trade there, but the order book behind it may be thin — only a few shares at that level before the price jumps. Placing a large market order into a shallow book sweeps through several levels and fills far worse than the top quote. On illiquid securities, check the depth, not just the top price.