A term used to describe the amount of liquidity in a particular equity (or group of equities) at a particular point in time. Often, as the "depth" or number of equities at play in the marketplace increases, traders are more likely to find sought after prices and size.
An ordered list of bid and sell orders; the ordering is usually with respect to price. The ability to see beyond the current bid and ask is a powerful tool when used by experienced traders.
In finance, market depth is the size of an order needed to move the market a given amount. If the market is deep, a large order is needed to change the price. Contrast with liquidity, the ease to find a trading partner for a given order.