OXFORD DICTIONARY
guerrilla, n. (also guerilla) a member of a small independently acting (usu. political) group taking part in irregular fighting, esp. against larger regular forces.
guerrilla war (or warfare) fighting by or with guerrillas.
Sp. guerrilla, dimin. of guerra war
THESAURUS
guerrilla
Charley, VC, Vietcong, bushfighter, bushwhacker, casual, guerilla, insurgent, irregular, maquis, maquisard, partisan, patriot, resistance, resistance fighter, saboteur, terrorist, underground, underground fighter