Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

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

         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).

         Fanatics are governed rather by imagination than by
         judgment.                                --Stowe.

Source : WordNet®

fanatic
     n : 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: {fiend}]

fanatic
     adj : marked by excessive enthusiasm for and intense devotion to a
           cause or idea; "rabid isolationist" [syn: {fanatical},
           {overzealous}, {rabid}]