En passant (from French: “in [the pawn's] 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 only moved one square forward. In this situation, the opposing pawn may, on the immediately subsequent move, capture the pawn as if taking it “as it passes” through the first square; the resulting position would then be 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.

Copyright Frank Johnson/Shootfilm.net

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. When claiming a draw by threefold repetition, two positions whose pieces are all on the same squares, with the same player to move, are considered different if there is the opportunity to make an en passant capture in one position but not the other.