Definition of Cossack
1
: a member of any of a number of autonomous communities drawn from various ethnic and linguistic groups (such as Slavs, Tatars, and Circassians) that formed in Ukraine, southern Russia, the Caucasus Mountains, and Siberia after about 1400 and that were completely incorporated into czarist Russia during the 18th and 19th centuries
2
: a mounted soldier serving in a unit drafted from Cossack communities
