From BR Bullpen
Every trip by a batter to the plate, including hits, walks, outs and reach by error, counts as a plate appearance. The only exception is when the time at bat is interrupted by an inning ending on a caught stealing or other similar event that prevents the completion of the batter's turn at the plate. Also, no plate appearance is counted when the batter reaches on catcher's interference, as he is not considered to have had a chance to swing the bat.
A plate appearance differs from an at bat, which excludes outcomes such as a base on balls or a hit by pitch. The number of at bats is used to calculate batting average and slugging percentage, while the number of plate appearances is used to calculate a batter's on-base percentage.
Theoretically, a plate appearance could last forever, as foul balls can prolong it until a ball is put in play, or a ball or strike is called. In practice, it is extremely rare for a plate appearance to be longer than 15 pitches.
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 Further Reading
- Brian Yonushonis: "The Infinitely Long MLB Plate Appearance, SABR, Volume 40, Number 1 (Spring 2011), pp. 103-107.