In baseball statistics, a player is credited with a plate appearance (denoted by PA) each time he completes a turn batting. A player completes a turn batting when: He strikes out or is declared out before reaching first base; or He reaches first base safely or is awarded first base (by a base on balls, hit by pitch, or catcher's interference); or He hits a fair ball which causes a preceding runner to be put out for the third out before he himself is put out or reaches first base safely (see also left on base, fielder's choice, force play)
A batter does not have a plate appearance if, while he was at bat, a preceding runner is put out on the basepaths for the third out in a way other than by the batter putting the ball into play (i.e., picked off, caught stealing). In this case, the same batter continues his turn batting in the next inning with no balls or strikes against him.
A batter may or may not be charged with a plate appearance (and possibly at-bat) in the rare instance when he is replaced by a pinch hitter after having already started his turn at bat. In this case, the pinch hitter would receive the plate appearance (and potential of an at-bat) unless the original batter is replaced when having 2 strikes against him and the pinch hitter subsequently completes the strikeout. In this case the plate appearance and at-bat are charged to the first batter. (see rule 10.15b)
PA = AB + BB + HBP + SH + SF + Times Reached on Defensive Interference
Basically, "plate appearances" = at bats + some of the scenarios excluded from at bats such as base on balls, hit by pitch, sacrifice or catcher's interference which positively affect the offensive team.
In common terminology, the term "at bat" is sometimes used to mean "plate appearance" (for example, "he fouled off the ball to keep the at bat alive"). The intent is usually clear from the context, although the term "official at bat" is sometimes used to explicitly refer to an at bat as distinguished from a plate appearance. However, terms such as turn at bat or time at bat are synonymous with plate appearance.
Section 10 of the official rules states what an at bat is not: "Number of times batted, except that no time at bat shall be charged when a player: (i) hits a sacrifice bunt or sacrifice fly; (ii) is awarded first base on four called balls; (iii) is hit by a pitched ball; or (iv) is awarded first base because of interference or obstruction "
The main use of the plate appearance statistic is in determining a player's eligibility for leadership in some offensive statistical categories, notably batting average; currently, a player must have 3.1 PAs per game scheduled to qualify for the batting title (for the 162-game schedule, that means 502 PAs). Also, it is often erroneously cited that total plate appearances is the divisor (i.e., denominator) used in calculating on base percentage (OBP), an alternative measurement of a player's offensive performance; in reality, the OBP denominator does not include certain PAs, such as sacrifice hits and times reached via either catcher’s interference or fielder’s obstruction.
Plate appearances are also used by scorers for "proving" a box score. If the game has been scored correctly, the total number of plate appearances for a team should equal the total of that team's runs, men left on base, and men put out.
Jimmy Rollins owns the major league record for plate appearances in a season with 778 in 2007.