En passant is a special pawn capture that can occur when an opponent's pawn moves two squares forward from its initial position. It can be captured as if it had only moved one square forward. En passant captures are notated by specifying the capturing pawn's file, followed by 'x', the destination square (not the square of the captured pawn), and 'e.p.' indicating the capture was en passant. For example, exd6e.p. denotes an en passant capture.