Cossacks (Ukrainian: козаки́, kozaky, Russian: казаки́, kazaki) are a group of predominantly East Slavic people who originally were members of democratic, semi-military communities in what is today Ukraine and Russia. They inhabited sparsely populated areas and islands in the lower Dnieper[1] and Don basins, and played an important role in the historical development of both Ukraine and Russia.

The Don Cossack Host, which had been established by the 16th century,[3] allied itself with the Tsardom of Russia. Together they began a systematic conquest and colonisation of lands in order to secure the borders on the Volga, the whole of Siberia (see Yermak Timofeyevich), the Yaik and the Terek Rivers. By the 18th century, Cossack hosts in the Russian Empire
served as buffer zones on her borders. However, the expansionist
ambitions of the empire relied on ensuring the loyalty of Cossacks,
which caused tension with their traditional freedom and independence. In
the 17th and 18th centuries this resulted in anti-imperial rebellions
and wars led by Stenka Razin, Kondraty Bulavin and Yemelyan Pugachev.
In extreme cases, whole Hosts could be dissolved, as was the fate of
the Zaporozhian Sich in 1775. By the end of the 18th century, Cossacks
were transformed into a special social estate (Sosloviye); they served as border guards on national and internal ethnic borders (as was the case in the Caucasus War) and regularly supplied men to conflicts such as the numerous Russo-Turkish Wars.
In return, they enjoyed vast social autonomy. This caused them to form a
stereotypical portrayal of 19th century Russian Empire abroad and her
government domestically.

During the Russian Civil War, Cossack regions became centres for the Anti-Bolshevik White movement, a portion of whom would form the White emigration. The Don and Kuban Cossacks even formed short-lived independent states, the Don Republic and the Kuban People's Republic, respectively. With the victory of the Red Army, the Cossack lands were subjected to famine, and suffered extensive repression. After the dissolution of the Soviet Union,
the Cossack lifestyle and its ideas have made a return in Russia. In
Russia's 2010 Population Census, Cossacks have been recognized as an ethnicity.[4] There are Cossack organizations in Russia, Kazakhstan, Ukraine, and USA.[