The Collaborative International Dictionary of English v.0.48:

Equestrian \E*ques"tri*an\, a. [L. equester, from eques
   horseman, fr. equus horse: cf. F. ['e]questre. See Equine.]
   1. Of or pertaining to horses or horsemen, or to
      horsemanship; as, equestrian feats, or games.
      [1913 Webster]

   2. Being or riding on horseback; mounted; as, an equestrian
      statue.
      [1913 Webster]

            An equestrian lady appeared upon the plains.
                                                  --Spectator.
      [1913 Webster]

   3. Belonging to, or composed of, the ancient Roman equities
      or knights; as, the equestrian order. --Burke.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Equestrian \E*ques"tri*an\, n.
   One who rides on horseback; a horseman; a rider.
   [1913 Webster]

WordNet (r) 3.0 (2006):

equestrian
    adj 1: of or relating to or composed of knights
    2: of or relating to or featuring horseback riding
    n 1: a man skilled in equitation [syn: horseman, equestrian,
         horseback rider]