Definition
An order book is an organized set of current buy and sell instructions, commonly grouped by price and displayed quantity for a market.
In market context
The best bid and ask appear at the top, while deeper levels show additional stated interest at less competitive prices. Displayed depth is not fixed: orders can execute, cancel, change, remain hidden, or exist on other venues. The book therefore offers a snapshot of available instructions rather than a guarantee that the shown quantity will remain when another order arrives.
Source
Use the primary source for fuller regulatory or market context.
