The Collaborative International Dictionary of English v.0.48:

Spectator \Spec*ta"tor\, n. [L. spectator: cf. F. spectateur.
   See Spectacle.]
   One who on; one who sees or beholds; a beholder; one who is
   personally present at, and sees, any exhibition; as, the
   spectators at a show. "Devised and played to take
   spectators." --Shak.
   [1913 Webster]

   Syn: Looker-on; beholder; observer; witness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

spectator
    n 1: a close observer; someone who looks at something (such as
         an exhibition of some kind); "the spectators applauded the
         performance"; "television viewers"; "sky watchers
         discovered a new star" [syn: spectator, witness,
         viewer, watcher, looker]
    2: a woman's pump with medium heel; usually in contrasting
       colors for toe and heel [syn: spectator pump, spectator]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "spectator":
   accepter, acquirer, addressee, aficionado, attender, attestant,
   attestator, attester, audience, auditor, auditory, beholder, buff,
   bystander, claqueur, cojuror, compurgator, congregation, consignee,
   deadhead, deponent, earwitness, eyewitness, fan, frequenter,
   gallery, gazer, getter, groundling, habitue, haunter, hearer,
   hired applauder, holder, house, informant, informer, listener,
   looker, looker-on, moviegoer, newer, observer, obtainer, onlooker,
   orchestra, pass holder, passerby, patron, payee, perceiver, pit,
   playgoer, procurer, receiver, recipient, seer, standee, swearer,
   taker, testifier, theater, theatergoer, trustee, viewer, visitor,
   voucher, watcher, witness