Bid-Ask Spread
The amount by which the ask price exceeds the bid.
Investopedia Commentary
For example, if the bid price is $20 and the ask price is $21 then the "bid-ask spread" is $1.
Related Links
Stock Basics Tutorial
Getting to Know Stock Exchanges
See also: Ask, Bid, Crossed Market, Inside Quote, Locked Market, Market Maker Spread, Market Versus Quote, Spread