In baseball, a player who bats; more generally, someone who strikes or hits things.
From 'hit' plus the agent suffix '-er' (one who does). The baseball usage became prominent in American English during the late 1800s as baseball terminology spread.
In baseball statistics, a 'hitter' is valued not just on how often they hit the ball, but on their 'batting average'—a percentage that mysteriously tops out around .333 even for the greatest players, suggesting something mathematical about how pitchers and batters compete!
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