In baseball

Baseball

Baseball is a bat-and-ball sport played between two teams of nine players each. The aim is to score runs by hitting a thrown ball with a bat and touching a series of four bases arranged at the corners of a ninety-foot diamond...

, a sacrifice fly is a batted ball that satisfies four criteria:

• There are fewer than two outs when the ball is hit.
• The ball is hit to the outfield
  Outfield
  The outfield is a sporting term used in cricket and baseball to refer to the area of the field of play further from the batsman or batter than the infield...
  
  .
• The batter is put out because an outfielder (or an infielder
  Infielder
  An infielder is a baseball player stationed at one of four defensive "infield" positions on the baseball field.-Standard arrangement of positions:In a game of baseball, two teams of nine players take turns playing offensive and defensive roles...
  
   running in the outfield) catches the ball on the fly
  Types of batted balls in baseball
  In baseball, a batted ball is any ball that, after a pitch, is contacted by the batter's bat. One or more of several terms are used to describe a batted ball, depending on how it comes off the bat and where in the field it lands....
  
   (or the batter would have been out if not for an error
  Error (baseball)
  In baseball statistics, an error is the act, in the judgment of the official scorer, of a fielder misplaying a ball in a manner that allows a batter or baserunner to reach one or more additional bases, when such an advance would have been prevented given ordinary effort by the fielder.The term ...
  
  ).
• A runner who is already on base scores on the play.

It is called a "sacrifice" fly because the batter presumably intends to cause a teammate to score a run, while sacrificing his own ability to do so.
As addressed within Rule 10.08(d) of the Official Baseball Rules, a sacrifice fly is not counted as a turn at bat

At bat

In baseball, an at bat or time at bat is used to calculate certain statistics, including batting average, on base percentage, and slugging percentage. It is a more restricted definition of a plate appearance...

 for the batter, though the batter is credited with a run batted in

Run batted in

Runs batted in or RBIs is a statistic used in baseball and softball to credit a batter when the outcome of his at-bat results in a run being scored, except in certain situations such as when an error is made on the play. The first team to track RBI was the Buffalo Bisons.Common nicknames for an RBI...

.

The purpose of not counting a sacrifice fly as an at-bat is to avoid penalizing hitters for a successful maneuver.