Source : Webster's Revised Unabridged Dictionary (1913)

Postilion \Pos*til"ion\, n. [F. postillon, It. postiglione, fr.
   posta post. See {Post} a postman.]
   One who rides and guides the first pair of horses of a coach
   or post chaise; also, one who rides one of the horses when
   one pair only is used. [Written also {postillion}.]

Source : WordNet®

postilion
     n : someone who rides the near horse of a pair in order to guide
         the horses pulling a carriage (especially a carriage
         without a coachman) [syn: {postillion}]