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
See en passant in American Heritage Dictionary 4
(äɴˌ pä-säɴˈ)
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.