A baseball and softball term.
An at-bat (AKA AB) is a baseball player's turn to bat. Each player takes a turn (except sometimes the pitcher, who may have a designated hitter, and anyone else who might need a pinch hitter).
When talking about a player's statistics (specifically batting average), an at-bat is a subset of plate appearance. Each time you go up to the plate is a plate appearance, but there are a number of cases in which an plate appearance won't count as a at-bat, and therefore not be counted in the batting average.
An at-bat happens when:
The batter reaches first base with a hit or,
the batter reaches first base on an error or,
the batter is called out for any reason or,
there is a fielder's choice.
An at-bat isn't counted when:
He receives a base on balls (BB) or,
He is hit by a pitch (walked) (HBP) or,
He hits a sacrifice fly or a sacrifice bunt or,
He is awarded first base due to interference or obstruction (usually by the catcher) or,
The inning ends while he is still at-bat or,
He is replaced by another hitter before his at-bat is completed (unless he is replaced with two strikes and his replacement strikes out).