Definitions
1.nounA move in which a pawn captures an opposing pawn on the same rank immediately after the latter has moved forward two squares on its first move in the game, as if it had moved forward only one square.
2.adverbIn passing, by the way, incidentally.
3.adverbOf a player's pawn when it has moved forward two squares on its first move in the game: captured "in passing" by the other player's pawn, as if the first player's pawn had only moved forward one square.