En passant

En passant (from French: in passing) is a move in chess. It is a special pawn capture, that can only occur immediately after a pawn moves two ranks forward from its starting position and an enemy pawn could have captured it had the pawn moved only one square forward (the capturing pawn must be on its fifth rank prior to executing this maneuver). The opponent captures the just-moved pawn "as it passes" through the first square. The resulting position is the same as if the pawn had moved only one square forward and the enemy pawn had captured it normally.

The en passant capture must be made at the very next turn or the right to do so is lost. It is the only occasion in chess in which a piece is captured but is not replaced on its square by the capturing piece. Like any other move, 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.

The en passant capture rule was added in the 15th century when the rule that gave pawns an initial double-step move was introduced. It prevents a pawn from using the two-square advance to pass an adjacent enemy pawn without the risk of being captured.

A pawn on its fifth rank may capture an enemy pawn on an adjacent file that has moved two squares in a single move, as if the pawn had moved only one square. The conditions are:

En passant is a unique privilege of pawns: pieces cannot capture en passant. It is the only occasion in chess in which the capturing piece does not replace the captured piece on its square.

There are some examples of en passant in chess openings. In this line from Petrov's Defence, White can capture the pawn on d5 en passant on his sixth move.

Another example occurs in the French Defense after 1.e4 e6 2.e5, a move once advocated by Wilhelm Steinitz. If Black responds with 2...d5, White can capture the pawn en passant with 3.exd6. Likewise, White can answer 2...f5 with 3.exf6e.p.

...
Wikipedia