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.

Source: Webster's Revised Unabridged Dictionary (1913)