en pas·sant 
(äɴ′ pä-säɴ)
(äɴ′ pä-säɴ)adv.
1. In passing; by the way; incidentally.
2. 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.
[French : en, in + passant, passing.]