At bat should not be confused with Plate appearance.
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. Plate Appearances = At Bats + Bases on Balls (Walks) + Hit By Pitch + Sacrifice Hits + Sacrifice Flies + Times Reached on Defensive Interference. A batter has an at bat every time he faces a pitcher, except in the following circumstances:
Section 10 of the official rules, available at MLB.com, states what an at bat is: "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
"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.