From Wikipedia:

The infield fly rule applies only when there are fewer than two outs, and there is a force play at third or home (runners on first and second base, or bases loaded). In these situations, if a fair fly ball is a hit that, in the umpire's judgment, is catchable by an infielder with ordinary effort, the batter is out regardless of whether the ball is actually caught in flight. The rule states that the umpire is supposed to announce, "Infield fly, if fair." If the ball will be almost certainly fair, the umpire will likely yell, "Infield fly, batter's out!" or just "Batter's out!" Umpires also typically raise one arm straight up to signal to everyone that the rule is in effect.

The Wikipedia reference is http://en.wikipedia.org/wiki/Infield_Fly_Rule.

Last updated on 17-August-09 by Forest Baseball