The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Guerrilla \Guer*ril"la\, a.
   Pertaining to, or engaged in, warfare carried on irregularly
   and by independent bands; as, a guerrilla party; guerrilla
   warfare.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guerrilla
    n 1: a member of an irregular armed force that fights a stronger
         force by sabotage and harassment [syn: guerrilla,
         guerilla, irregular, insurgent]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "guerrilla":
   Charley, VC, Vietcong, bushfighter, bushwhacker, casual, guerilla,
   insurgent, irregular, maquis, maquisard, partisan, patriot,
   resistance, resistance fighter, saboteur, terrorist, underground,
   underground fighter