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:
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 speficially 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[.]"
An at bat is counted when:
Pete Rose had 14,053 career at bats, the all-time major league and National League record. The American League record is held by Carl Yastrzemski, whose 11,988 career at bats were all in the AL.
The single season record is held by Jimmy Rollins, who had 716 at bats in 2007; Willie Wilson, Ichiro Suzuki and Juan Samuel also had more than 700 at bats in a season. 14 players share the single game record of 11 at bats in a single game, all of which were extra inning games. In games of 9 innings or fewer, the record is 7 at bats and has occurred more than 200 times.
"At bat", "up", "up at bat", and "at the plate" are all phrases describing a batter who is facing the pitcher. Note that just because a player is described as being "at bat" in this sense, he will not necessarily be given an at bat in his statistics; the phrase actually signifies a plate appearance (assuming it is eventually completed). This ambiguous terminology is usually clarified by context. To refer explicitly to the technical meaning of "at bat" described above, the term "official at bat" is sometimes used.
Official Baseball Rule 5.06(c) provides that "[a] batter has legally completed his time at bat when he is put out or becomes a runner" (emphasis added). The "time at bat" defined in this rule is more commonly referred to as a plate appearance, and the playing rules (Rules 1 through 8) uses the phrase "time at bat" in this sense.[note 2] In contrast, the scoring rules use the phrase "time at bat" to refer to the statistic at bat, defined in Rule 9.02(a)(1), but sometimes uses the phrase "official time at bat" or refers back to Rule 9.02(a)(1) when mentioning the statistic. The phrase "plate appearance" is used in Rules 9.22 and 9.23 dealing with batting titles and hitting streaks, but is not defined anywhere in the rulebook.