The Collaborative International Dictionary of English v.0.48:

Enthusiasm \En*thu"si*asm\, n. [Gr. ?, fr. ? to be inspired or
   possessed by the god, fr. ?, ?, inspired: cf. enthousiasme.
   See Entheal, Theism.]
   1. Inspiration as if by a divine or superhuman power;
      ecstasy; hence, a conceit of divine possession and
      revelation, or of being directly subject to some divine
      impulse.
      [1913 Webster]

            Enthusiasm is founded neither on reason nor divine
            revelation, but rises from the conceits of a warmed
            or overweening imagination.           --Locke.
      [1913 Webster]

   2. A state of impassioned emotion; transport; elevation of
      fancy; exaltation of soul; as, the poetry of enthusiasm.
      [1913 Webster]

            Resolutions adopted in enthusiasm are often repented
            of when excitement has been succeeded by the wearing
            duties of hard everyday routine.      --Froude.
      [1913 Webster]

            Exhibiting the seeming contradiction of
            susceptibility to enthusiasm and calculating
            shrewdness.                           --Bancroft.
      [1913 Webster]

   3. Enkindled and kindling fervor of soul; strong excitement
      of feeling on behalf of a cause or a subject; ardent and
      imaginative zeal or interest; as, he engaged in his
      profession with enthusiasm.
      [1913 Webster]

            Nothing great was ever achieved without enthusiasm.
                                                  --Emerson.
      [1913 Webster]

   4. Lively manifestation of joy or zeal.
      [1913 Webster]

            Philip was greeted with a tumultuous enthusiasm.
                                                  --Prescott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

enthusiasm
    n 1: a feeling of excitement
    2: overflowing with eager enjoyment or approval [syn:
       exuberance, enthusiasm, ebullience]
    3: a lively interest; "enthusiasm for his program is growing"

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "enthusiasm":
   acquiescence, activity, agreeability, agreeableness, alacrity,
   amenability, amusement, animation, ardency, ardor, avidity, brio,
   briskness, bug, calenture, cathexis, cheerful consent, compliance,
   concern, concernment, consent, cooperativeness, craze, crazy fancy,
   curiosity, devotedness, devotion, diversion, docility, eagerness,
   earnest, earnestness, ebullience, elan, enthusiasticalness,
   excitement, exuberance, fad, fanaticism, fascination,
   favorable disposition, favorableness, fervency, fervor, fieriness,
   fire, forwardness, furor, furore, gameness, glow, goodwill, gusto,
   hobby, hurrah, impassionedness, impetuosity, impetus, infatuation,
   interest, joie de vivre, keenness, life, liveliness, lustiness,
   mania, manic-depressive psychosis, matter of interest, mettle,
   passion, pastime, perkiness, pertness, pliability, pliancy,
   promptness, rage, readiness, receptive mood, receptiveness,
   receptivity, relish, responsiveness, right mood, robustness,
   special interest, spirit, spiritedness, tractability,
   ungrudgingness, unloathness, unreluctance, vehemence, vivacity,
   warmth, willing ear, willing heart, willingness, zeal, zealousness,
   zest, zestfulness, zing



The Devil's Dictionary (1881-1906):

ENTHUSIASM, n.  A distemper of youth, curable by small doses of
repentance in connection with outward applications of experience.
Byron, who recovered long enough to call it "entuzy-muzy," had a
relapse, which carried him off -- to Missolonghi.