The Collaborative International Dictionary of English v.0.48:

Insurgent \In*sur"gent\, a. [L. insurgens, p. pr. of insurgere
   to rise up; pref. in- in + surgere to rise. See Surge.]
   Rising in opposition to civil or political authority, or
   against an established government; insubordinate; rebellious.
   "The insurgent provinces." --Motley.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Insurgent \In*sur"gent\, n. [Cf. F. insurgent.]
   A person who rises in revolt against civil authority or an
   established government; one who openly and actively resists
   the execution of laws; a rebel.

   Syn: See Rebel.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insurgent
    adj 1: in opposition to a civil authority or government [syn:
           insurgent, seditious, subversive]
    n 1: a person who takes part in an armed rebellion against the
         constituted authority (especially in the hope of improving
         conditions) [syn: insurgent, insurrectionist, freedom
         fighter, rebel]
    2: 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:

36 Moby Thesaurus words for "insurgent":
   agitator, anarch, anarchist, brawler, breakaway, contumacious,
   extreme, extremist, extremistic, factious, frondeur, insubordinate,
   insurrectionary, insurrectionist, insurrecto, malcontent, maverick,
   mutineer, mutineering, mutinous, nonconformist, rebel, rebellious,
   revolter, revolutional, revolutionary, revolutionist, rioter,
   riotous, seditionary, seditious, subversive, traitor, traitorous,
   treasonable, turbulent



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSURGENT. One who is concerned in an insurrection. He differs from a rebel
in this, that rebel is always understood in a bad sense, or one who unjustly
opposes the constituted authorities; insurgent may be one who justly opposes
the tyranny of constituted authorities. The colonists who opposed the
tyranny of the English government were insurgents, not rebels.