Capturing a pawn that moved 2 spaces with a pawn that could have captured it if it had only moved 1 space, on the next turn only.

French "in passing." It occurs when a pawn moves two squares from its starting position, and passes an enemy pawn that has advanced to its fifth rank. The advanced pawn on the fifth rank may choose to capture the pawn as if the pawn had only moved forward one square. This capture must be made immediately after the two square advance, or else the right to capture "en passant" is lost. In chess notation an en passant capture is labelled "e.p."