Peasant-soldiers in Ukraine and in several regions of the former Russian Empire who, until 1918, held certain privileges in return for rendering military service.
The first Cossack companies were formed in the 15th century, when Ukraine, then part of the unified Polish-Lithuanian state, took independent measures to defend itself against the devastating Tatar raids. The Ukrainian Cossacks, of heterogeneous background, were chiefly Russians and Poles and included many runaway serfs. By the 16th century they had settled along the lower and middle Dnepr River (see Zaporozhie Cossacks).
Similar communities grew up on the Don (see Don Cossacks) and its tributaries. They were all organised on principles of political and social equality, and originally were virtually autonomous. Each community elected an ataman as its head, while an assembly of all the Cossacks chose the hetman.
The Cossacks gave shelter to refugees from Poland and Russia and took part in peasant revolts in Ukraine and Russia in the 17th and 18th centuries. Open struggle ensued between the Cossacks and the Polish and Russian governments.
By the late 18th century the Cossacks had lost most of their political autonomy and had been made the privileged military class, integrated with the Russian military forces. Under the last tsars they were often used to quell strikes and other disturbances.
The primary unit of Cossack organisation, the village, was largely self-governed until 1918. Land was held in common by the village. But an 1869 law, which allowed officers and civil servants to own land as personal property, contributed to the break-up of the traditional cohesiveness of Cossack village life.
In the 19th century the Russian government began to organise new Cossack units so that by the early 20th century there were 11 Cossack communities, each named for its location - Don, Kuban, Terek, Astrakhan, Ural, Orenburg, Siberia, Semirechensk, Transbaikalia, Amur, and Ussuri.
Following the Bolshevik Revolution, the majority of the Cossacks fought against the Soviet armies in the civil war of 1918-1920. In 1920 the Soviet government abolished all their privileges and between 1928 and 1933 the Cossack communities were forcibly collectivised.
In 1936, however, the Cossack party regained status, being allowed to form several cavalry divisions in the Russian army. Although the Cossack communities were incorporated into the Soviet administrative system, their traditions and customs continue to survive, notably on the Don and Kuban rivers.