It's when the batter in baseball swings at a pitch, but fails to hit it, when a batter does not swing at a pitch that is thrown within the strike zone, when the ball is hit foul and the strike count is less than 2 (a batter cannot strike out on a foul ball, however he can fly out), when a ball is bunted foul, regardless of the strike count, when the ball touches the batter as he swings at it, when the ball touches the batter in the strike zone, or when the ball is a foul tip.