From the Spanish for 'little war', the word first described the hit-and-run rebels who fought against the installation of Napoleon's brother Joseph on the throne of Spain in 1808. Guerrilla warfare later became the main feature of the conflicts in such countries as Vietnam and Algeria.

Guerillas is a 1975 novel by V. S. Naipaul.

an excellent anarchist punk band

Combatants that are members of an organized and recognized military force whose activities normally are directed to harassing, delaying, or disrupting opposing forces; they normally wear civilian clothes.

"A member of an irregular military force operating usually in small, independent groups capable of great speed and mobility." (From The American Heritage Dictionary)