The Collaborative International Dictionary of English v.0.48:

Fad \Fad\ (f[a^]d), n. [Cf. Faddle.]
   1. A hobby; freak; whim.
      [1913 Webster]

            It is your favorite fad to draw plans. --G. Eliot.
      [1913 Webster]

   2. a practise followed enthusiastically by a number of people
      for a limited period of time; as, the latest fad in
      fashion.

   Syn: craze; mania.
        [PJC] -- Fad"dist, n.
        -- Fad"dish, a.
        [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "fad":
   best seller, big hit, boutade, brainstorm, brilliant success,
   capriccio, caprice, chic, conceit, crank, craze, crazy idea,
   crotchet, cry, faddiness, faddishness, faddism, faddist, fancy,
   fantastic notion, fantasy, flimflam, fool notion, freak,
   freakish inspiration, furore, gas, gasser, great success,
   harebrained idea, hit, humor, killing, kink, maggot, megrim,
   meteoric success, mode, momentary success, notion, novelty,
   passing fancy, quirk, rage, resounding triumph, riot,
   roaring success, sensation, smash, smash hit, style, toy, triumph,
   vagary, vogue, whim, whim-wham, whimsy, wow, wrinkle