Collaborationism is cooperation with enemy forces against one's country. Legally, it may be considered as a form of treason.^{[citation needed]} Collaborationism may be associated with criminal deeds in the service of the occupying power, which may include complicity with the occupying power in murder, persecutions, pillage, and economic exploitation or participation in a puppet government.

The term collaborate dates from 1871, and is a back-formation from collaborator (1802), from the French collaborateur as used during the Napoleonic Wars against smugglers trading with England and assisting in the escape of monarchists, and is itself derived from the Latin collaboratus, past participle of collaborare "work with", from com- "with" + labore "to work." The meaning of "traitorous cooperation with the enemy" dates from 1940, originally in reference to the Vichy Government of France and those who cooperated with or helped the Nazis, following the French defeat in the Battle of France.

In France, a distinction emerged between the collaborateur and the collaborationniste. The latter expression is mainly used to describe individuals enrolled in pseudo-Nazi parties, often based in Paris, who had an overwhelming belief in fascist ideology or were simply anti-communists.Collaborateurs on the other hand, could engage in collaboration for a number of more pragmatic reasons, such as preventing infrastructure damage for use by the occupation forces or personal ambition, and were not necessarily believers in fascism per se. Arch-collaborators like Pierre Laval or René Bousquet are thus distinct from collaborationists.