A Cossack host or Cossack voisko (Казачье войско, kazachye voysko, sometimes incorrectly translated as Cossack Army) was an administrative subdivision of Cossacks in Imperial Russia. It consisted of a certain territory with Cossack settlements that had to provide military regiments for service in the Russian Imperial Army and for border patrol. Usually the hosts were named after the regions of their dislocation. The stanitsa, or village formed the primary unit of this organization.
Cossack voiskos on Russian soil were disbanded in 1920, at the end of the Russian Civil War. Those Cossacks who settled abroad continued to preserve the traditions of their hosts (i.e. the Triunited Don-Kuban-Terek Cossack Union).
In the Russian Empire, the Cossacks constituted eleven separate hosts, settled along the frontiers: the Don Cossack Host, Kuban Cossack Host, Terek Cossack Host, Astrakhan Cossack Host, Ural Cossack Host, Orenburg Cossack Host, Siberian Cossack Host, Semiryechye Cossack Host, Transbaikal Cossack Host, Amur Cossack Host, and Ussuri Cossack Host. There was also a small number of the Cossacks in Krasnoyarsk and Irkutsk, who would form the Yenisey Cossack Host and Irkutsk Cossack Regiment of the Ministry of the Interior in 1917.