[Alberta's Child]

Actually, here's the definition of an Infield Fly quoted directly from the official rules of Major League Baseball (I bolded the item for line drives):

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.

[FredZarguna]

What’s considered a line drive is entirely at the umpire’s discretion.