a rule that applies to pawns… and kings
En passant is one of the last move innovations introduced in chess. It was first applied in the 14th-15th century, a bit after the two-square first move for pawns was implemented.
The idea of the move is the following. When a pawn tries to make a two square move it is considered that it goes square by square. If it falls under attack by another pawn while it passes the first square, the pawn can be captured “in passing”. Here is what FIDE rule 3.7 d. states:
“A pawn attacking a square crossed by an opponent`s pawn which has advanced two squares in one move from its original square may capture this opponent`s pawn as though the latter had been moved only one square. This capture is only legal on the move following this advance and is called an `en passant` capture. This move must be made in the event that no other legal move is possible.”
En passant idea exists in other elements of the game. For example a king cannot castle through check (FIDE rule 3.8 ii.). Since the king can move one square at a time, it cannot cross a square under attack. That means the castle move is prevented by “in passing”, although the term is not used in that case.
If you are looking for alternative definitions of en-passant do not forget to check out our dictionary.