The Collaborative International Dictionary of English v.0.48:

Fanaticism \Fa*nat"i*cism\, n. [Cf. Fanatism.]
   Excessive enthusiasm, unreasoning zeal, or wild and
   extravagant notions, on any subject, especially religion,
   politics or ideology; religious frenzy.

   Syn: See Superstition.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fanaticism
    n 1: excessive intolerance of opposing views [syn: fanaticism,
         fanatism, zealotry]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "fanaticism":
   agitation, authoritarianism, balkiness, bias, bibliolatry, bigotry,
   blind side, blind spot, blinders, bluster, brawl, broil, brouhaha,
   bullheadedness, cacophony, chaos, charismatic gift,
   charismatic movement, charismatic renewal, closed mind, commotion,
   cramped ideas, dedication, determination, devotedness, devotion,
   disturbance, doggedness, dogmatism, ebullition, embroilment,
   enthusiasm, excessiveness, extravagance, extremeness, extremism,
   fanaticalness, ferment, fervor, fixed mind, flap, fomentation,
   foofaraw, franticness, frenzy, fume, furor, furore, fury, fuss,
   gift of tongues, glossolalia, hardheadedness, headstrongness,
   hideboundness, hubbub, hysteria, illiberality, infatuation,
   inflexible will, insularism, insularity, intolerance,
   little-mindedness, littleness, madness, mania, mean mind, meanness,
   monomania, mulishness, narrow sympathies, narrow views,
   narrow-mindedness, narrowness, nearsightedness, obduracy,
   obsessiveness, obstinacy, obstinateness, odium theologicum,
   opinionatedness, overambitiousness, overanxiety, overanxiousness,
   overdevoutness, overeagerness, overenthusiasm, overpiousness,
   overreaction, overreligiousness, overrighteousness,
   overzealousness, pandemonium, parochialism, partiality, passion,
   pentecostalism, perfervidness, perseverance, pertinacity,
   pettiness, petty mind, pigheadedness, prejudice, provincialism,
   purblindness, rabidness, racket, rage, restiveness, revival,
   revivalism, row, ruckus, rumpus, sanctimony, self-will,
   shortsightedness, shut mind, single-mindedness, smallness,
   stiff neck, stiff-neckedness, storminess, straitlacedness,
   strongheadness, stubbornness, stuffiness, sulkiness, sullenness,
   tempestuousness, tenaciousness, tenacity, tumult, tumultuousness,
   turbulence, turmoil, ultrazealousness, uncatholicity,
   uncooperativeness, unregenerateness, uproar, upset, wildness,
   willfulness, zeal, zealotism, zealotry, zealousness