order book
Beginner
A list of everyone who wants to buy or sell something, and at what price.
Novice
For each traded instrument, the list of outstanding buy orders (bids) and sell orders (offers) by price. A trading system keeps its own copy, updated from the exchange’s messages.
Expert
Often kept in aggregated form, as price levels with total quantity at each. In hardware the challenge is finding an instrument’s record quickly: a large book doesn’t fit on chip, so lookups go to external SRAM through a hash table.
Explained in Fintech and trading (Industries).