Sacrifice fly

A sacrifice fly is a fly ball hit deep enough that a runner on third base can tag up and score after the catch, and it's one of the rare outs in baseball that actually helps the batting team. The batter is credited with an RBI but, unusually, isn't charged with an official at-bat for it, since the sport doesn't want to penalize a hitter's batting average for a productive out that did exactly its job. It only counts as a sacrifice fly if a run actually scores, so a deep fly with a runner on third who doesn't tag up, or can't score, is scored as a plain flyout instead. The rule's been on and off the books since the 1890s and settled into its current form in 1954. Full history at SABR.