What does "guerrilla" mean?

Guerrilla: 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. Note: The term guerrilla is the diminutive of the Spanish word guerra, war, and means petty war, that is, war carried on by detached parties; generally in the mountains. . . . A guerrilla party means, an irregular band of armed men, carrying on an irregular war, not being able, according to their character as a guerrilla party, to carry on what the law terms a regular war. F. Lieder.