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 their 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 they accumulate 502 plate appearances during the season.

Batters will not receive credit for an at bat if their plate appearances end under the following circumstances:

• They receive a base on balls (BB).
• They are hit by a pitch (HBP).
• They hit a sacrifice fly or a sacrifice bunt (also known as sacrifice hit).
• They are awarded first base due to interference or obstruction, usually by the catcher.
• They are replaced by another hitter before their at bat is completed, in which case the plate appearance and any related statistics go to the pinch hitter (unless they are replaced with two strikes and their 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 they are 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.

Put shortly, an at bat is a specific type of plate appearance in which in the batter puts the ball in play intending to get on base. This is why at bats, and not plate appearances, are used to calculate batting average, as plate appearances in general can result in many outcomes that don't involve the ball being put in play, and batting average specifically measures a batter's contact hitting.

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[.]"