En passant

En passant (, "in passing") is a move in chess. It is a special pawn that can only occur immediately after a pawn makes a move of two squares from its starting square, and it could have been captured by an enemy pawn had it advanced only one square. The opponent captures the just-moved pawn "as it passes" through the first square. The result is the same as if the pawn had advanced only one square and the enemy pawn had captured it normally.
The "en passant" capture must be made on the very next turn or the right to do so is lost. "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.
Conditions.
A pawn on its fifth may capture an enemy pawn on an adjacent 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—other pieces cannot capture "en passant". It is the only capture in chess in which the capturing piece does not replace the captured piece on its square.
Notation.
In either algebraic or descriptive chess notation, "en passant" captures are sometimes denoted by "e.p." or similar, but such notation is not required. In algebraic notation, the capturing move is written as if the captured pawn advanced only one square; for example, ...bxa3 or ...bxa3e.p. (see beginning illustration).
Examples.
In the opening.
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 Defence 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.exd6e.p. Likewise, White can answer 2...f5 with 3.exf6e.p.
An example is from this game by Steinitz and Bernhard Fleissig.
Unusual examples.
In the diagram, if Black plays 1...g5+, it seems to checkmate White, but it is in fact a blunder. Black overlooks that White can counter this check with the "en passant" capture 2.fxg6e.p., which cross-checks and checkmates Black. This game is a draw if neither side errs.
Another example is this game between Gunnar Gundersen and Albert H. Faul. Black has just moved his pawn from f7 to f5. White could capture the f-pawn "en passant" with his e-pawn, but instead played:
The bishop on c1 effects a discovered check. 14...Kh7 results in 15.Qxg7#.
The "en passant" capture and discovered checks place Black in checkmate (from White's rook on h1, even without help from White's bishop). An "en passant" capture is the only way a double check can be delivered without one of the checking pieces moving, as in this case.
The largest known number of "en passant" captures in one game is three, shared by three games; in none of them were all three captures by the same player. The earliest known example is a 1980 game between Alexandru Sorin Segal and Karl Heinz Podzielny.
In chess compositions.
"En passant" captures have often been used as a theme in chess compositions, as they "produce striking effects in the opening and closing of lines". In the 1938 composition by Kenneth S. Howard, the move 1. d4 introduces the threat of 2.d5+ cxd5 3.Bxd5#. Black can capture the d4-pawn "en passant" in either of two ways:
An example showing the effect "en passant" captures have on pins is this 1902 composition by Sommerfeldt:
The move 
threatens 2.Qf2#. The moves of the black e-pawn are restricted in an unusual manner. The "en passant" capture 1...exd3e.p.+ is illegal (it exposes the king to check), but 
is legal. This, however, removes the black king's access to e3, allowing 
Historical context.
Allowing the "en passant" capture, together with the introduction of the two-square first move for pawns, was one of the last major rule changes in European chess, and occurred between 1200 and 1600. In most places the "en passant" rule was adopted at the same time as allowing the pawn to move two squares on its first move, but it was not universally accepted until the Italian rules were changed in 1880.
The motivation for "en passant" was to prevent the newly added two-square first move for pawns from allowing a pawn to evade capture by an enemy pawn.
In chess variants.
In most chess variants, if pawns are allowed to take two steps forward on their first move, the "en passant" rule is the same as in standard chess. Some larger variants allow pawns to make an initial move of "more" than two squares (such as the 16×16 game Chess on a Really Big Board, in which pawns may make up to six steps forward) − in such games "en passant" captures are usually allowed on any square the given pawn had just passed. 
In variants with more than two spatial dimensions, "en passant" rules vary. In some three-dimensional chess variants, such as Millennium or Alice chess, "en passant" is allowed as long as the pawn's two-step move was not a purely vertical move. In 5D Chess with Multiverse Time Travel, "en passant" (and castling) are allowed in the normal spacial dimensions, but are not allowed across multiverses or across time.
Since the "en passant" rule relies on pawns advancing by more than one square, any chess variants that do not allow the initial two-step option for pawns (such as Dragonchess or Raumschach) also do not feature "en passant" captures in their set of rules. This includes all Asian variants, which only allow pawns to move one step at a time and thus cannot be captured "en passant" (in fact, pawns in "shogi", "xiangqi" and "janggi" cannot even capture diagonally at all).
Threefold repetition and stalemate.
The possibility of an "en passant" capture is relevant to the claim of a draw by threefold repetition. Two positions with the same configuration of pieces, with the same player to move, are for this purpose considered different if there was an opportunity to make an "en passant" capture in the first position, and that opportunity no longer exists in the second position.
In his book on chess organization and rules, International Arbiter Kenneth Harkness wrote that it is frequently asked if an "en passant" capture must be made if it is the only legal move. This point was debated in the 19th century, with some arguing that the right to make an "en passant" capture is a "privilege" that one cannot be compelled to exercise. In his 1860 book "Chess Praxis", Howard Staunton wrote that the "en passant" capture is mandatory in that instance.
The rules of chess were amended to make this clear. Today, it is settled that the player must make that move (or resign). The same is true if an "en passant" capture is the only move to get out of check.
References.
Bibliography