Real Dictionary

Cossack in American English
a member of any of several groups of peasants, chiefly of Russian and Polish descent, that lived in autonomous communal settlements, esp. in the Ukraine, until the late 19th cent: in return for special privileges, they served in the cavalry under the Czar
of or characteristic of the Cossacks or their culture
noun: a person belonging to any of certain groups of Slavs living chiefly in the southern part of Russia in Europe and forming an elite corps of horsemen

Cossack in British English
adjective: of, relating to, or characteristic of the Cossacks
noun: (formerly) any of the free warrior-peasants of chiefly East Slavonic descent who lived in communes, esp in Ukraine, and served as cavalry under the tsars