THE INFIELD FLY RULE
Rule 2.00 of the Official Baseball Rules (bold added for emphasis):
An INFIELD FLY is a fair fly ball (not including a line drive nor an attempted bunt) which can be caught by an infielder with ordinary effort, when first and second, or first, second and third bases are occupied, before two are out. The pitcher, catcher and any outfielder who stations himself in the infield on the play shall be considered infielders for the purpose of this rule. The ball is alive and runners may advance at the risk of the ball being caught, or retouch and advance after the ball is touched, the same as on any fly ball. ... The umpire's judgment must govern, and the decision should be made immediately.