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.
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            The sudden apparition of the Spaniards. --Prescott.
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            The apparition of Lawyer Clippurse occasioned much
            speculation in that portion of the world. --Sir W.
                                                  Scott.
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   2. The thing appearing; a visible object; a form.
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            Which apparition, it seems, was you.  --Tatler.
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   3. An unexpected, wonderful, or preternatural appearance; a
      ghost; a specter; a phantom. "The heavenly bands . . . a
      glorious apparition." --Milton.
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            I think it is the weakness of mine eyes
            That shapes this monstrous apparition. --Shak.
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   4. (Astron.) The first appearance of a star or other luminary
      after having been invisible or obscured; -- opposed to
      occultation.
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   Circle of perpetual apparition. See under Circle.
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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"