Plate Appearance and at Bat
Every trip to the plate is a plate appearance. Only some of them are at bats, and the difference is where a lot of statistics hide.
What it actually is. A plate appearance is any completed turn at bat. An at bat is a plate appearance minus the ones that are not judged to be the hitter's chance to hit: walks, hit by pitches, sacrifice bunts, sacrifice flies and catcher's interference. So a man who walks four times has four plate appearances and zero at bats, and his batting average is unchanged.
What somebody who knows the game gets out of it. This is the most important small distinction in baseball statistics and it explains most of the twentieth century's blind spot. Batting average divides by at bats, so it cannot see a walk at all: two hitters with identical averages can differ by eighty points of on base percentage. On base percentage divides by plate appearances precisely to fix that, which is why it is the number that correlates with scoring runs and average is not. Any time you see a leaderboard, check the denominator, because that choice is the argument.
The numbers worth knowing. A full time player gets roughly 600 to 700 plate appearances and roughly 550 to 600 at bats in a season. The qualifying line for a batting title is 502 plate appearances, which is 3.1 per team game.
What people get wrong about it. A sacrifice fly is not an at bat but a sacrifice fly does count against on base percentage, which is one of the strange corners of the rulebook. A sacrifice bunt is excluded from both. There is no consistent principle there, only history.
At Little League and travel ball. In a continuous batting order every player on the roster gets plate appearances, so youth statistics are far more evenly spread than professional ones and far noisier. Ninety plate appearances over a travel season is about a seventh of a major league sample, which is why a .450 hitter and a .300 hitter on the same 12U team may not actually be different hitters.
Batter's Eye: look up anything in baseball