A sacrifice fly occurs when a batter hits a fly-ball out to the outfield or foul territory that allows a runner to score. The batter is given credit for an RBI. (If the ball is dropped for an error but it is determined that the runner would have scored with a catch, then the batter is still credited with a sacrifice fly.)
A sacrifice fly does not count as an at-bat and therefore does not count against a player's batting average. The thinking behind the rule is that with a man on third base and fewer than two outs, a batter will often intentionally try to hit a fly ball, sacrificing his time at bat to help score a run. However, sacrifice flies count against a player's on-base percentage.
The sacrifice fly was adopted as an official rule in 1954, at which point it was distinguished from the sacrifice bunt. Before 1954, Major League Baseball went back and forth as to whether a sacrifice fly should be counted statistically. In the years that it was counted (1908-31 and '39), it was grouped together with the sacrifice bunt as simply a "sacrifice."
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