The French phrase en passant means 'in passing'. It is used in much the same way as 'by the way' - "he mentioned Dickens, en passant, to show off his knowledge of literature." It seems less affected to use the equivalent English phrase in passing - except im the area where it may be most commonly used, among chess players. Here it means
- En passant (from French: "in passing") is a maneuver [sic] in chess which is performed after a player moves a pawn two squares forward from its starting position, and an opposing pawn captures it as if it had only moved one square. En passant may only be played immediately after a two-square square pawn advance, or the right to capture "in passing" is lost.
- After pawns were granted the ability to move two squares on their first move, the En passant rule was introduced in 1490 to prevent pawns from having too much power or freedom. (from Chessopedia, [], accessed 26/08/2014.)
Note that if you choose to use the French phrase other than for chess - in ordinary writing academic English - you should italicize the phrase. Chess writers normally do not.