guerrilla

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A member of an irregular, usually indigenous military or paramilitary force operating in small bands in occupied territory to harass and undermine the enemy, as by surprise raids.
• adjective Of or relating to guerrillas or their tactics.
• adjective Using unconventional and usually inexpensive means to generate interest or raise concern among the public, as in commercial marketing or social activism.

from The Century Dictionary.

• noun War carried on by the repeated attacks of independent bands; a system of irregular warfare by means of raids and surprises.
• noun Properly, a band of independent and generally predatory fighters in a war; now, more commonly, an individual member of such a band.
• Of or pertaining to guerrillas: as, a guerrilla attack; a guerrilla band.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Pertaining to, or engaged in, warfare carried on irregularly and by independent bands
• noun An irregular mode of carrying on war, by the constant attacks of independent bands, adopted in the north of Spain during the Peninsular war.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A non-official war carried out by small independent groups; a guerrilla war.
• noun A soldier in a small independent group, fighting against the government or regular forces by surprise raids.
• adjective military Relating to, using, or typical of guerrilla warfare, or its principles of small independent or non-official perpetrators.
• adjective marketing Relating to, using, or typical of guerrilla marketing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a member of an irregular armed force that fights a stronger force by sabotage and harassment