Source : Webster's Revised Unabridged Dictionary (1913)

Guerrilla \Guer*ril"la\, n. [Sp., lit., a little war, skirmish,
   dim. of guerra war, fr. OHG. werra discord, strife. See
   {War}.]
   1. An irregular mode of carrying on war, by the constant
      attacks of independent bands, adopted in the north of
      Spain during the Peninsular war.

   2. 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.

Guerrilla \Guer*ril"la\, a.
   Pertaining to, or engaged in, warfare carried on irregularly
   and by independent bands; as, a guerrilla party; guerrilla
   warfare.

Source : WordNet®

guerrilla
     n : a member of an irregular armed force that fights a stronger
         force by sabotage and harassment [syn: {guerilla}, {irregular},
          {insurgent}]

guerrilla
     adj : used of independent armed resistance forces; "guerrilla
           warfare"; "partisan forces" [syn: {guerrilla(a)}, {guerilla(a)},
            {underground}, {irregular}]