From WordNet (r) 3.0 (2006) [wn]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
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2. Being or riding on horseback; mounted; as, an equestrian
statue.
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An equestrian lady appeared upon the plains.
--Spectator.
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3. Belonging to, or composed of, the ancient Roman equities
or knights; as, the equestrian order. --Burke.
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From The Collaborative International Dictionary of English v.0.48 [gcide]:
Equestrian \E*ques"tri*an\, n.
One who rides on horseback; a horseman; a rider.
[1913 Webster]