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The title was used during the Provisional Government for regional heads of administration, but it is mostly associated with a number of Cheka and military functions in Bolshevik and Soviet government military forces during the Russian Civil War (the White Army widely used the collective term "bolsheviks and commissars" for their opponents) and with the later terms People's Commissar (or narkom) for government ministers and political commissar in the military.
It is based on similar titles in a variety of languages (such as commissaire in French, Kommissar in German) most often attached to a criminal investigator in the police; they are usually translated as "commissioner".
People's Commissar 
A People's Commissar (informally abbreviated narkom) was a government official serving in a Council of People's Commissars. This title was first used by the Russian SFSR (out of dislike for the tsarist and "bourgeois" term minister) and then copied among the many Soviet and Bolshevik-controlled states in the Russian Civil War.
The government departments headed by a People's Commissar were called People's Commissariat (informally abbreviated narkomat).
People's Commissars and People's Commissariats were renamed Ministers and Ministries in 1946 by a decree of the Supreme Soviet of the Soviet Union.
Political commissar 
A political commissar was a high-ranking functionary at a military headquarters who held coequal rank and authority with the military commander of the unit. Political commissars were established to control the military forces by the Communist party. No military order might be issued which did not have the prior approval of both the commander and the commissar.
Although lower-level political officers never received the same military training as commanding officers, most commissars were high-ranking party bosses and never had any military training or talent.
Following the disasters of 1942, the political command was abolished. Political officers only survived at the regimental level, in the form of a Deputy for Political Matters, and at the front level, where they formed the Military Councils with respective military commanders.
Military commissar 
The voenkom (Russian: военком), translated as war commissar, is the head of a military commissariat - a regional office that drafts men for military service, executes plans for military mobilization, and maintains records on military reserves.
Until the late 1930s, the People's Militsiya and Internal Troops of the NKVD had no personal ranks, and used many various position-ranks instead. In 1935, the Militsiya created a special system of personal ranks that was a blend of standard military ranks and position-ranks; this system was largely reused by the newly-created Main Directorate of State Security (GUGB) in their rank structure, although they had Commissar-style ranks for top officers in place of Militsiya-style inspector and director.
From 1943, the Militsiya switched to a new rank system and insignia introduced in the Soviet Army. Instead of General ranks, top officers used Commissar of Militsiya 3rd, 2nd, and 1st rank, even though they used army-standard Major General, Lieutenant General and Colonel General shoulder boards. These Commissar ranks were replaced by corresponding General ranks in 1975.
The GUGB also switched to military-style ranks and insignia in 1945, although they replaced Commissar-style ranks with General officer ranks right away.
Other uses 
A similar term in French describes the equivalent of the rank of Major both in the army of the Ancien Régime and the French Revolution. Such officials were not military officers but reported back to the political authorities: the king and the National Assembly (French Revolution), respectively.
Various historical German states have used an equivalent title, Reichskommissar (a compound of Reich and the German Kommissar), for several administrators who held responsibility over a territory or area of government.