Source : Webster's Revised Unabridged Dictionary (1913)

En passant \En` pas`sant"\ [F.]
   In passing; in the course of any procedure; -- said specif.
   (Chess), of the taking of an adverse pawn which makes a first
   move of two squares by a pawn already so advanced as to
   threaten the first of these squares. The pawn which takes en
   passant is advanced to the threatened square.

Source : WordNet®

en passant
     adv : incidentally; in the course of doing something else; "he
           made this remark in passing" [syn: {in passing}]