A person taking part in irregular fighting by small groups acting independently. The word was coined during the Peninsular War (1807–14) to describe the Spanish partisans fighting the armies of Napoleon. From Spain the use of the word spread to South America and thence to the USA.

Guerrilla warfare avoids full-scale military confrontation while keeping the enemy under pressure with many small-scale skirmishes. The technique is suited to harsh terrain, particularly jungle and mountainous areas, and has been used effectively by materially weak forces against militarily strong opponents, where there are few opportunities for conventional military forces to use superior firepower. During World War II guerrillas formed the basis of the resistance movements that harassed the Japanese and German occupying forces. In post-war years they have become associated with such revolutionary movements as those in South America under Che Guevara. See also terrorism.