En passant (from French: in passing) is a move in the board game of chess. En passant is a special capture made immediately after a player moves a pawn two squares forward from its starting position, and an opposing pawn could have captured it as if it had moved only one square forward. In this situation, the opposing pawn may capture the pawn as if taking it "as it passes" through the first square. The resulting position is the same as if the pawn had only moved one square forward and the opposing pawn had captured normally. The en passant capture must be done on the very next turn, or the right to do so is lost. Such a move is the only occasion in chess in which a piece captures but does not move to the square of the captured piece. If an en passant capture is the only legal move available, it must be made. En passant capture is a common theme in chess compositions.
This rule was added in the 15th century when the rule giving pawns the option of initially moving two squares was introduced. The rationale is so that a pawn cannot pass by another pawn using the two-square move without the risk of it being captured.