Market Depth
A measure of how much can be traded before the price moves by a set amount, usually read from resting orders near the quote.
Data providers commonly report depth as the total value of resting orders within a fixed distance of the mid price, such as within one or two percent on each side. Depth is a better description of tradable size than volume, because volume counts what already happened while depth describes what is currently available. It is measured separately on each side, since a market can be deep in one direction and shallow in the other. Depth also collapses fastest exactly when it matters, because quoting firms widen or withdraw during sharp moves.
In practice
Two venues can report similar daily volume while one shows several times more resting size within one percent of the mid price.
The common misunderstanding
Depth is a snapshot, not a promise; the orders counted in it can be canceled in the moment they would otherwise be needed.