in passing; by the way: used, in chess, of the capture of a pawn which has just taken a first move of two squares, passing an opponent's pawn that dominates the first of those squares and that makes the capture by moving to that square
Origin:
Fr
See en passant in American Heritage Dictionary 4
adverb
In passing; by the way; incidentally.
Used in reference to a move in chess in which a pawn that has just completed an initial advance to its fourth rank is captured by an opponent pawn as if it had only moved to its third rank.