1. [noun] A member or descendant of an originally (semi-)nomadic population of Eastern Europe and the adjacent parts of Asia, that eventually settled in parts of the Polish-Lithuanian Commonwealth, the Russian tsarist Empire (where they constituted a legendary military caste) and the Soviet Union, particularly in areas now comprising southern Russia and Ukraine.
2. [noun] A member of a military unit (typically cavalry, originally recruited exclusively from the above)
3. [noun] Alternative letter-case form of Cossack
4. [noun] A member of a Slavic people living in southern European Russia and Ukraine and adjacent parts of Asia and noted for their horsemanship and military skill; they formed an elite cavalry corps in czarist Russia.
