What is the official definition of en passant?

Answer 1

The term is 'capturing en passant.' (It's a French term.) It arises when a pawn (obviously one that has not previously moved) moves two spaces ahead, and in doing so ends up immediately beside an opponent's pawn. The opponent's pawn has the option of capturing en passant by removing the first pawn and placing his pawn in the square that the first pawn moved through. The positioning after each player has moved would be the same as if the first pawn had only moved one square forward, and then was captured in normal fashion by the opponent's pawn.

Answer 2

En passant is a special pawn capture in Chess. The literal meaning of en passant is "passing."