Guerrilla warfare is a form of irregular warfare and refers to conflicts in which a small group of combatants including, but not limited to, armed civilians (or "irregulars") use military tactics, such as ambushes, sabotage, raids, the element of surprise, and extraordinary mobility to harass a larger and less-mobile traditional army, or strike a vulnerable target, and withdraw almost immediately.

The term means "little war" in Spanish, and the word, guerrilla, has been used to describe the concept since the 18th century, and perhaps earlier.