When it is a batter’s turn in the lineup, the announcer on TV might say that he is taking ‘his turn at bat.’ In this context, the term at bat is being used loosely, but what the batter is actually doing is making a plate appearance.
A plate appearance is a batter’s time at the plate regardless of the outcome—outs, walks, home runs are all plate appearances. We also track a stat called at bats that makes more sense when it comes to calculating certain averages (like batting average).
An at bat is a plate appearance except those resulting in sacrifice bunts, sacrifice flies, walks, hit batsmen, and obstruction or interference.
Why the different categories?
When it comes to calculating a statistic like batting average, which is the one of the most common ways to rate a player’s offensive performance, it is unfair to penalize or give the batter the benefit of the doubt in some of his plate appearances’ outcomes.
Batting average is calculated by dividing the number of hits by the number of at bats over a span of time or within a particular set of circumstances. If we were to use plate appearances rather than at bats, it would essentially penalize a batter for the times that he walked or was hit by a pitch. This would not make sense as these outcomes are positive from the batter’s perspective.
Batting average is displayed by rounding the decimal to three places. We would say a hitter with a .310 batting average “is batting three-ten.” Even though this means that nearly 70% of this batter’s at bats result in an out, when you take into account every player’s average a .310 hitter would likely be one of the best in the league.