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In baseball, an at bat (AB) or time at bat is used to calculate certain statistics, including batting average, on base percentage, and slugging percentage. It is a more restricted definition of a plate appearance. A batter starts with an at bat every time he faces a pitcher; however, the batter gets "no time at bat" in 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 hit (also known as sacrifice bunt).
- He is awarded first base due to interference or obstruction, usually by the catcher.
- The inning ends while he is still at bat (due to the third out being made by a runner caught stealing, for example). In this case, the batter will come to bat again in the next inning, though he now has no balls or strikes on him.
- He is replaced by another hitter before his at bat is completed (unless he is replaced with two strikes and his replacement strikes out).
Section 10.02.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: (1) hits a sacrifice bunt or sacrifice fly; (2) is awarded first base on four called balls; (3) is hit by a pitched ball; or (4) is awarded first base because of interference or obstruction..."
An at bat is counted when:
- The batter reaches first base on a hit
- The batter reaches first base on an error
- The batter is called out for any reason other than as part of a sacrifice
- There is a fielder's choice
At bat as a phrase 
"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.
- In 1887, Major League Baseball counted bases on balls as hits. The result was high batting averages, including some near .500, and the experiment was abandoned the following season.
- "Rule 10.01: The Rules of Scoring" (PDF). Official Baseball Rules. Commissioner of Baseball, Major League Baseball. Archived from the original on 19 March 2010. Retrieved 2010-03-25.