An insurgency is a violent, armed rebellion by small, lightly armed bands who practice guerrilla warfare from primarily rural base areas against a larger authority. The key descriptive feature of insurgency is its asymmetric nature: small irregular forces face a large, well-equipped, regular military force state adversary. Due to this asymmetry, insurgents avoid large-scale direct battles, opting instead to blend in with the civilian population (mainly in the countryside) where they gradually expand territorial control and military forces. Insurgency frequently hinges on control of and collaboration with local populations.

An insurgency can be fought via counter-insurgency warfare, as well as other political, economic and social actions of various kinds. Due to the blending of insurgents with the civilian population, insurgencies tend to involve considerable violence against civilians (by the state and the insurgents). State attempts to quell insurgencies frequently lead to the infliction of indiscriminate violence, whereas rebel control of territory frequently involves violence against the civilian population.

Where a revolt takes the form of armed rebellion, it may not be viewed as an insurgency if a state of belligerency exists between one or more sovereign states and rebel forces. For example, during the American Civil War, the Confederate States of America was not recognized as a sovereign state, but it was recognized as a belligerent power, and thus Confederate warships were given the same rights as United States warships in foreign ports.

Sometimes there may be one or more simultaneous insurgencies (multipolar) occurring in a country. The Iraq insurgency is one example of a recognized government versus multiple groups of insurgents. Other historic insurgencies, such as the Russian Civil War, have been multipolar rather than a straightforward model made up of two sides. During the Angolan Civil War there were two main sides: MPLA and UNITA. At the same time, there was another separatist movement for the independence of the Cabinda region headed up by FLEC. Multipolarity extends the definition of insurgency to situations where there is no recognized authority, as in the Somali Civil War, especially the period from 1998 to 2006, where it broke into quasi-autonomous smaller states, fighting among one another in changing alliances.