The Collaborative International Dictionary of English v.0.48:

Fanatic \Fa*nat"ic\, a. [L. fanaticus inspired by divinity,
   enthusiastic, frantic, fr. fanum fane: cf. F. fanatique. See
   Fane.]
   Pertaining to, or indicating, fanaticism; extravagant in
   opinions; ultra; unreasonable; excessively enthusiastic,
   especially on religious subjects; as, fanatic zeal; fanatic
   notions.
   [1913 Webster]

         But Faith, fanatic Faith, once wedded fast
         To some dear falsehood, hugs it to the last. --T.
                                                  Moore.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fanatic \Fa*nat"ic\, n.
   A person affected by excessive enthusiasm, particularly on
   religious subjects; one who indulges wild and extravagant
   notions of religion.
   [1913 Webster]

         There is a new word, coined within few months, called
         fanatics, which, by the close stickling thereof,
         seemeth well cut out and proportioned to signify what
         is meant thereby, even the sectaries of our age.
                                                  --Fuller
                                                  (1660).
   [1913 Webster]

         Fanatics are governed rather by imagination than by
         judgment.                                --Stowe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fanatic
    adj 1: marked by excessive enthusiasm for and intense devotion
           to a cause or idea; "rabid isolationist" [syn: fanatic,
           fanatical, overzealous, rabid]
    n 1: a person motivated by irrational enthusiasm (as for a
         cause); "A fanatic is one who can't change his mind and
         won't change the subject"--Winston Churchill [syn:
         fanatic, fiend]

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "fanatic":
   Christian, God-fearing man, accepter, addict, aficionado, alien,
   aliene, ass, bedlamite, believer, bigot, bitter-ender,
   borderline case, buff, bug, bullethead, case, catechumen,
   character, chauvinist, churchgoer, churchite, churchman, collector,
   communicant, convert, crackbrain, crackpot, crank,
   daily communicant, dement, demon, demoniac, devotee, devotionalist,
   diehard, disciple, doctrinaire, dogmatist, donkey, eager beaver,
   eccentric, energumen, enthusiast, extremist, faddist, fan,
   fanatico, fiend, flake, follower, fou, freak, good Christian,
   great one for, hardnose, hermit, hobbyist, hobo, hound, idiot,
   illiberal, infatuate, intolerant, intransigeant, intransigent,
   jingo, kook, last-ditcher, lone wolf, loner, loon, loony, lunatic,
   lunatic fringe, madman, male chauvinist, maniac, maverick,
   meshuggenah, monomaniac, mule, natural, neophyte, noncompos,
   nonconformist, nut, odd fellow, oddball, oddity, original,
   outsider, pariah, perverse fool, phrenetic, pietist, pig, pighead,
   positivist, proselyte, purist, pursuer, queer duck, queer fish,
   queer specimen, rabid, racist, radical, rara avis, raving lunatic,
   receiver, religionist, revolutional, revolutionary, revolutionist,
   rhapsodist, saint, screwball, sexist, solitary, standpat,
   standpatter, stickler, sucker for, superpatriot, theist, tramp,
   truster, type, ultra, ultraist, ultranationalist, visionary,
   votary, weirdo, zealot