Depth of market (DOM) is a measure of the supply and demand for liquid, tradeable assets. It is based on the number of open buy and sell orders for a given asset such as a stock or futures contract. The greater the quantity of those orders, the deeper or more liquid, the market is considered to be.
Depth of market data is also known as the order book since it consists of a list of pending orders for a security or currency. The data in the book is used to determine which transactions can be processed. DOM data is available from most online brokers for free or for a small fee.