WordNet (r) 3.0 (2006):

extremism
    n 1: any political theory favoring immoderate uncompromising
         policies

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "extremism":
   Fabianism, Jacobinism, New Left, Old Left, abandon, anarchism,
   anarcho-syndicalism, bigotry, boundlessness, conversion,
   criminal syndicalism, egregiousness, enormousness, exaggeration,
   excess, excessiveness, exorbitance, exorbitancy, extravagance,
   extravagancy, extreme, extreme left, extreme left wing,
   extremeness, extremes, extremity, fabulousness, factiousness,
   fanaticalness, fanaticism, giantism, gigantism, gluttony,
   gradualism, hyperbole, hypertrophy, immoderacy, immoderateness,
   immoderation, incontinence, inordinacy, inordinance,
   inordinateness, insurgentism, insurrectionism, intemperance,
   intemperateness, left-wing extremism, meliorism, monstrousness,
   mutinousness, nihilism, nimiety, outrageousness, overdevelopment,
   overenthusiasm, overgreatness, overgrowth, overindulgence,
   overlargeness, overmuch, overmuchness, overreaction,
   overreligiousness, overzealousness, perfervidness, progressivism,
   rabidness, radical reform, radicalism, radicalization,
   rebelliousness, reform, reformation, reformism, regeneration,
   revisionism, revolution, revolutionism, riotousness,
   sans-culotterie, sans-culottism, sedition, seditiousness,
   subversiveness, syndicalism, too much, too-muchness,
   traitorousness, transformation, treasonableness, ultraconservatism,
   ultraism, ultrazealousness, unconscionableness, undueness,
   unreasonableness, unrestrainedness, utopianism, zealotism,
   zealotry