Definition of "Cossack"

A member or descendant of an originally (semi-)nomadic population of Eastern Europe and the adjacent parts of Asia, formed in part of runaways from neighbouring countries, that eventually settled in parts of the Polish-Lithuanian Commonwealth and the Russian tsarist Empire and constituted a military caste, particularly in areas now comprising southern Russia and Ukraine.  examples

A member of a military unit (typically cavalry, originally recruited exclusively from the above).  examples

(obsolete) A Ukrainian.

(derogatory) A mercenary; a regular or irregular soldier used to oppress a minority, such as in anti-Jewish pogroms; a police officer, particularly one used in strike-breaking; a violent thug.  examples