An irregular mode of carrying on war, by the constant attacks of independent bands, adopted in the north of Spain during the Peninsular war. [1913 Webster]
One who carries on, or assists in carrying on, irregular warfare; especially, a member of an independent band engaged in predatory excursions in war time. [1913 Webster]
, a.
Pertaining to, or engaged in, warfare carried on irregularly and by independent bands; as, a guerrilla party; guerrilla warfare. [1913 Webster]
, n. (also guerilla) a member of a small independently acting (usu. political) group taking part in irregular fighting, esp. against larger regular forces.
Idiom
guerrilla war (or warfare) fighting by or with guerrillas.