"En passant"

This is a special chess rule which applies only to the movement of pawns.

The term "en passant" comes from the French term "in passing".

Recall - The pawn moves forward one square at a time and cannot move backward (or retreat). 

The pawn may however, move two squares forward on its first move only (this may result in a special move played called "en passant").

The "en passant" rule applies when one's pawn reaches the 5th rank and an opponent's pawn moves two spaces (on its first move) to pass the pawn directly beside it. Only a pawn that occupies a space directly beside another may be taken "en passant", as if the opponent's pawn had moved only one space.

Also, "en passant" must be made immediately by the player on the following move.