Wiktionary
n. (context baseball English) A rule providing that a fair fly ball hit with a force play at third base, which is deemed catchable by an umpire, be ruled an out irrespective of the fielder play, eliminating the force play at all bases.
Wikipedia
The infield fly rule is a rule of baseball that treats certain fly balls as though caught, before the ball is caught and even if the infielder fails to catch it or drops it on purpose. The umpire's declaration of an infield fly means that the batter is out (and all force plays are removed) regardless of whether the ball is caught. The rule exists to prevent the defense from achieving a double play or triple play by failing to catch a ball that an infielder could catch with ordinary effort. The rule is limited to apply only in situations where this result is possible.
A ball batted into the air subjects baserunners to a dilemma. If the ball is caught, they must return to their original base; if not caught, the batter becomes a runner and certain runners are forced to advance to the next base. Baserunners study the fielder and advance only far enough from the base to ensure that they can return safely. If a presumed catch becomes a non-catch, forced runners must run forward instead of back. This creates an advantage for the defense in intentionally failing to execute an easy catch, which the infield fly rule exists to remove.
Usage examples of "infield fly rule".
But what would you say if I told you that letting them worry about building that nuclear plant in Bahrain, or declaring war against China, or finding a solution to the continuing abuse of the infield fly rule, would give us men eight more years of life?