The Collaborative International Dictionary of English v.0.48:

Cavalry \Cav"al*ry\, n. [F. cavalerie, fr. It. cavalleria. See
   Cavalier, and cf. chivalry.] (Mil.)
   That part of military force which serves on horseback.
   [1913 Webster]

   Note: Heavy cavalry and light cavalry are so
         distinguished by the character of their armament, and
         by the size of the men and horses.
         [1913 Webster]

WordNet (r) 3.0 (2006):

cavalry
    n 1: troops trained to fight on horseback; "500 horse led the
         attack" [syn: cavalry, horse cavalry, horse]
    2: a highly mobile army unit