En passant

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