The Collaborative International Dictionary of English v.0.48:

Hallucination \Hal*lu`ci*na"tion\ (-n[=a]"sh[u^]n), n. [L.
   hallucinatio: cf. F. hallucination.]
   1. The act of hallucinating; a wandering of the mind; error;
      mistake; a blunder.
      [1913 Webster]

            This must have been the hallucination of the
            transcriber.                          --Addison.
      [1913 Webster]

   2. (Med.) The perception of objects which have no reality, or
      of sensations which have no corresponding external cause,
      arising from disorder of the nervous system, as in
      delirium tremens; delusion.
      [1913 Webster]

            Hallucinations are always evidence of cerebral
            derangement and are common phenomena of insanity.
                                                  --W. A.
                                                  Hammond.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hallucination
    n 1: illusory perception; a common symptom of severe mental
         disorder
    2: a mistaken or unfounded opinion or idea; "he has delusions of
       competence"; "his dreams of vast wealth are a hallucination"
       [syn: delusion, hallucination]
    3: an object perceived during a hallucinatory episode; "he
       refused to believe that the angel was a hallucination"