The Collaborative International Dictionary of English v.0.48:

Apparition \Ap`pa*ri"tion\, n. [F. apparition, L. apparitio, fr.
   apparere. See Appear.]
   1. The act of becoming visible; appearance; visibility.
      --Milton.
      [1913 Webster]

            The sudden apparition of the Spaniards. --Prescott.
      [1913 Webster]

            The apparition of Lawyer Clippurse occasioned much
            speculation in that portion of the world. --Sir W.
                                                  Scott.
      [1913 Webster]

   2. The thing appearing; a visible object; a form.
      [1913 Webster]

            Which apparition, it seems, was you.  --Tatler.
      [1913 Webster]

   3. An unexpected, wonderful, or preternatural appearance; a
      ghost; a specter; a phantom. "The heavenly bands . . . a
      glorious apparition." --Milton.
      [1913 Webster]

            I think it is the weakness of mine eyes
            That shapes this monstrous apparition. --Shak.
      [1913 Webster]

   4. (Astron.) The first appearance of a star or other luminary
      after having been invisible or obscured; -- opposed to
      occultation.
      [1913 Webster]

   Circle of perpetual apparition. See under Circle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

apparition
    n 1: a ghostly appearing figure; "we were unprepared for the
         apparition that confronted us" [syn: apparition,
         phantom, phantasm, phantasma, fantasm, specter,
         spectre]
    2: the appearance of a ghostlike figure; "I was recalled to the
       present by the apparition of a frightening specter"
    3: something existing in perception only; "a ghostly apparition
       at midnight" [syn: apparition, phantom, phantasm,
       phantasma, fantasm, shadow]
    4: an act of appearing or becoming visible unexpectedly;
       "natives were amazed at the apparition of this white
       stranger"

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "apparition":
   Masan, appearance, appearing, arising, astral, astral spirit,
   avatar, banshee, bogey, brainchild, bubble, chimera, coming,
   coming into being, coming-forth, control, corposant, delirium,
   delusion, departed spirit, disclosure, disembodied spirit, duppy,
   dybbuk, eidolon, emergence, epiphany, exposure, false image, fancy,
   fantasque, fantasy, fiction, figment, figure, form, forthcoming,
   ghost, grateful dead, guide, hallucination, hant, haunt,
   idle fancy, idolum, illusion, image, imagery, imagination,
   imagining, immateriality, incarnation, incorporeal,
   incorporeal being, incorporeity, insubstantial image, invention,
   issuance, larva, lemures, maggot, make-believe, manes,
   manifestation, marshfire, materialization, materializing, mirage,
   myth, occurrence, oni, opening, phantasm, phantasma,
   phantasmagoria, phantom, phasm, phenomenon, poltergeist, presence,
   presentation, realization, revelation, revenant, rise, rising,
   romance, shade, shadow, shape, showing, showing forth,
   shrouded spirit, sick fancy, specter, spectral ghost, spectrum,
   spirit, spook, sprite, theophany, thick-coming fancies, trip,
   umbra, unfolding, unfoldment, unsubstantiality, vapor, vision,
   waking dream, walking dead man, wandering soul, whim, whimsy,
   wildest dream, wildest dreams, wraith, zombie