En passant capture
En passant capture is a special move of pawns in Chess. It can happen when a pawn makes a double move; then an opposing pawn can capture it on the very next move as if the first pawn moved only one field. If it does not capture it en passant immediately, later the possibility is lost.
The black pawn makes a double move, and passes the field where the white pawn could capture it normally.
Then the white pawn can capture it en passant on the next move. If it does not, it cannot do it later.
Captured it en passant. Note the speciality of the move: the white pawn moved to a different field where the black pawn was captured.
The en passant capture was introduced at the same time as the double move of pawns. The pawn's option to move two fields from its initial field was allowed to quicken opening, but on the other hand, this option would have ruined a general aspect of pawns: that a pawn cannot pass an opposing pawn without the possibility of being captured by it. To preserve this aspect, the en passant capture was introduced.
In chess variants there are similar rules for en passant capture than in chess, with the following differences (unless different is stated):