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"

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "hallucination":
   aberration, agnosia, apparition, bamboozlement, befooling, block,
   blocking, bluffing, brainchild, bubble, calculated deception,
   chimera, circumvention, conning, deceiving, deception,
   deceptiveness, defrauding, delirium, delirium tremens, delusion,
   delusion of persecution, delusiveness, disorientation, dream,
   dupery, eidolon, enmeshment, ensnarement, entanglement, entrapment,
   fallaciousness, fallacy, falseness, fancy, fantasque, fantasy,
   fata morgana, fiction, figment, flight of ideas, flimflam,
   flimflammery, fond illusion, fooling, ghost, hallucinosis,
   hoodwinking, idle fancy, illusion, imagery, imagination, imagining,
   insubstantial image, invention, kidding, maggot, make-believe,
   mental block, mental confusion, mind-expansion, mirage, myth,
   nihilism, nihilistic delusion, outwitting, overreaching, paralogia,
   phantasm, phantom, psychological block, putting on, romance,
   self-deception, sick fancy, snow job, song and dance, spoofery,
   spoofing, subterfuge, swindling, thick-coming fancies, trickiness,
   tricking, trip, tripping, vapor, victimization, vision, whim,
   whimsy, wildest dreams, willful misconception, wishful thinking,
   wraith



Bouvier's Law Dictionary, Revised 6th Ed (1856):

HALLUCINATION, med. jur. It is a species of mania, by which "an idea
reproduced by the memory is associated and embodied by the imagination."
This state of mind is sometimes called delusion or waking dreams.
     2. An attempt has been made to distinguish hallucinations from
illusions; the former are said to be dependent on the state of the
intellectual organs and, the latter, on that of those of sense. Ray, Med.
Jur. Sec. 99; 1 Beck, med. Jur. 538, note. An instance is given of a
temporary hallucination in the celebrated Ben Johnson, the poet. He told a
friend of his that he had spent many a night in looking at his great toe,
about which he had seen Turks and Tartars, Romans and Carthagenians, fight,
in his imagination. 1 Coll. on Lun. 34. If, instead of being temporary, this
affection of his mind had been permanent, he would doubtless have been
considered insane. See, on the subject of spectral illusions, Hibbert,
Alderson and Farrar's Essays; Scott on Demonology, &c.; Bostock's
Physiology, vol. 3, p. 91, 161; 1 Esquirol, Maladies Mentales, 159.