The Infield Fly Rule in baseball (specifically, rule 6.05e, coupled with the definition in rule section 2.00) is intended

to prevent unfair gamesmanship by the fielders that would result in an easy double play or triple play.
The rule
The infield fly rule applies when there are fewer than two outs, and there is a force play at third (runners on first and

second base, or bases loaded). In these situations, if a fair fly ball is 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.

Any fair fly ball that could have been caught by an infielder with ordinary effort is covered by the rule, regardless of

where the ball was caught. The ball need not be caught by an infielder, nor must it be caught in the infield. For

example, if an infielder retreats to the outfield in an effort to catch a fly ball with ordinary effort, the Infield Fly Rule

would be invoked, even if an outfielder ultimately caught the ball, and even if no infielder attempted to make a play

on the ball. Similarly, a fly ball within the infield that could have been caught by an infielder with ordinary effort, but is

caught by an outfielder, would also be covered by the rule.

On a caught infield fly, the runners must tag up (retouch their base at the time of pitch) in order to be eligible to

advance, as on any catch. If the infield fly falls to fair ground untouched, or is touched and dropped, runners need

not tag up. In either case, since the batter is out, the force play on other runners is removed.
History
This rule was introduced in 1895, in response to infielders intentionally dropping pop-ups in order to get multiple

outs by forcing out the runners on base, who were pinned near their bases while the ball was in the air.

It is a cousin to the third-strike rule. A batter who swings and misses a third strike which is not caught automatically

becomes a runner if first base is not occupied with fewer than two outs, and also if first base is occupied with two

outs. After 2005, the batter must be running towards first base before he departs the dirt circle area around home

plate or he is called out.
Misconceptions
Participants and fans sometimes misunderstand the infield fly rule. The infield fly rule is not in effect if there is a runner

on first only, as the rule-makers assumed fielders would not gain a significant advantage by forcing out the runner

rather than the batter; in either case, the net result would be one more out and a runner on first base. Also, an infield

fly does not affect baserunners other than the batter. Just like any other fly ball, if an infield fly is caught, runners must

retouch (or "tag up") their time-of-pitch base before attempting to advance; if an infield fly is not properly caught, no

tag up is required and the runners may try to advance.

The infield fly rule cannot be invoked on line drives or bunts; also, the infield fly rule is not intended to cover all

situations where the defense may wish to allow a fly ball to drop uncaught. For example, with just a runner on first,

an alert infielder might purposely let a popup drop to the ground and get the force at second, if it happens that the

runner on first is faster afoot than the batter-runner is, or if the batter is loafing on his way to first base. This is only

legal if the fielder lets the ball hit the ground untouched, which carries some risk to the fielder as it might bounce

away from him. However, in all situations where the infield fly rule does not apply, a different rule (6.05l) prevents

fielders from touching a catchable ball and dropping it intentionally in an attempt to turn a double or triple play.