En passant

An enemy pawn has just gone two squares at once, landing right beside yours. Had it moved only one square, you could have captured it. The rule says: you may capture it anyway.

Your pawn takes it en passant, moving to the square it skipped over. The enemy pawn is removed from the board, even though your pawn does not land on its square.

One condition, and it is strict: you must do it immediately, on the very next move. Play anything else and the chance is gone for good.