En passant

When a pawn advances two squares from its starting position and there is an opponent's pawn on an adjacent file next to its destination square, then the opponent's pawn can capture it en passant (in passing), and move to the square the pawn passed over. However, this can only be done on the very next move, otherwise the right to do so is forfeit. For example, if the black pawn has just advanced two squares from g7 (initial starting position) to g5, then the white pawn on f5 may take it via en passant on g6 (but only on white's next move).