(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
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
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. ] [ 1913 Webster ]