a member of a group of frontiersmen of southern Russia organized as cavalry in the czarist army

A privileged peasant soldier in tsarist Russia, preeminently a member of the cavalry and horse artillery. Prior to the 18th century, Cossacks were runaway peasants and others who had fled to the steppe region between the Dniepr and Don rivers and formed free, self-governing communes organized along military lines. The Cossack population constituted a separate class or estate in the Russian Empire.

The Cossacks are a traditional community of people living in the southern steppe regions of Eastern Europe and Asian Russia. They are famous for their self-reliance and military skills, particularly horsemanship. "Cossack" may also refer to a member of a Cossack military unit.