The Cossacks are a predominantly East Slavic Orthodox Christian people originating in the Pontic–Caspian steppe of Ukraine and southern Russia. Historically, they were a semi-nomadic and semi-militarized people, who, while under the nominal suzerainty of various Eastern European states at the time, were allowed a great degree of self-governance in exchange for military service. Although numerous linguistic and religious groups came together to form the Cossacks, most of them coalesced and became East Slavic-speaking Orthodox Christians. The Cossacks were particularly noted for holding democratic traditions. The rulers of the Polish-Lithuanian Commonwealth and Russian Empire endowed Cossacks with certain special privileges in return for the military duty to serve in the irregular troops (mostly cavalry). The various Cossack groups were organized along military lines, with large autonomous groups called hosts. Each host had a territory consisting of affiliated villages called stanitsa.

They inhabited sparsely populated areas in the Dnieper, Don, Terek, and Ural river basins, and played an important role in the historical and cultural development of both Russia and Ukraine.

The Cossack way of life persisted into the twentieth century, though the sweeping societal changes of the Russian Revolution disrupted Cossack society as much as any other part of Russia; many Cossacks migrated to other parts of Europe following the establishment of the Soviet Union, while others remained and assimilated into the Communist state. Cohesive Cossack-based units were organized and many fought for both Nazi Germany and the Soviet Union during World War II.

After World War II, the Soviet Union disbanded the Cossack units in the Soviet Army, and many of the Cossack traditions were suppressed during the years of rule under Joseph Stalin and his successors. During the Perestroika era in the Soviet Union in the late 1980s, descendants of Cossacks moved to revive their national traditions. In 1988, the Soviet Union passed a law allowing the re-establishment of former Cossack hosts and the formation of new ones. During the 1990s, many regional authorities agreed to hand over some local administrative and policing duties to their Cossack hosts.

Between 3.5 and 5 million people associate themselves with the Cossack cultural identity across the world. Cossack organizations operate in Russia, Ukraine, Belarus, Kazakhstan, Canada, and the United States.