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 WordNet 3.0 & Open English WordNet

The Cossacks are a predominantly East Slavic Orthodox Christian people originating in the Pontic–Caspian steppe of eastern Ukraine and southern Russia. Wikipedia

A Slavic-speaking social group of Eastern Europe Wikipedia Disambiguation

East Slavic-speaking Orthodox Christian people originating in the Pontic steppe Wikidata

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 the neighbouring countries, that eventually settled in parts of the Polish-Lithuanian Commonwealth and the Russian tsarist Empire (where they constituted a legendary military caste), particularly in areas now comprising southern Russia and Ukraine. Wiktionary

Member of a population. Wiktionary (translation)