In baseball, an at bat (AB) or time at bat is a batter's turn batting against a pitcher. An at bat is different from a plate appearance. A batter is credited with a plate appearance regardless of what happens during his turn at bat, but a batter is credited with an at bat only if that plate appearance does not have one of the results enumerated below. While at bats are used to calculate certain statistics, including batting average and slugging percentage, a player can qualify for the season-ending rankings in these categories only if he accumulates 502 plate appearances during the season.
A batter will not receive credit for an at bat if his plate appearance ends under the following circumstances:
- He receives a base on balls (BB).
- He is hit by a pitch (HBP).
- He hits a sacrifice fly or a sacrifice bunt (also known as sacrifice hit).
- He is awarded first base due to interference or obstruction, usually by the catcher.
- He is replaced by another hitter before his at bat is completed, in which case the plate appearance and any related statistics go to the pinch hitter (unless he is replaced with two strikes and his replacement completes a strikeout, in which case the at bat and strikeout are still charged to the first batter).
In addition, if the inning ends while he is still at bat (due to the third out being made by a runner caught stealing, for example), no at bat or plate appearance will result. In this case, the batter will come to bat again in the next inning, though the count will be reset to no balls and no strikes.
Rule 9.02(a)(1) of the official rules of Major League Baseball defines an at bat as: "Number of times batted, except that no time at bat shall be charged when a player: (A) hits a sacrifice bunt or sacrifice fly; (B) is awarded first base on four called balls; (C) is hit by a pitched ball; or (D) is awarded first base because of interference or obstruction[.]"