The Collaborative International Dictionary of English v.0.48:

Cossack \Cos"sack\ (k?s"s?k), n. [Russ. kozak', kazak': cf.
   Turk. kaz[=a]k.]
   One of a warlike, pastoral people, skillful as horsemen,
   inhabiting different parts of the Russian empire and
   furnishing valuable contingents of irregular cavalry to its
   armies, those of Little Russia and those of the Don forming
   the principal divisions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Cossack
    n 1: a member of a Slavic people living in southern European
         Russia and Ukraine and adjacent parts of Asia and noted for
         their horsemanship and military skill; they formed an elite
         cavalry corps in czarist Russia