The Collaborative International Dictionary of English v.0.48:

Illusion \Il*lu"sion\, n. [F. illusion, L. illusio, fr.
   illudere, illusum, to illude. See Illude.]
   1. An unreal image presented to the bodily or mental vision;
      a deceptive appearance; a false show; mockery;
      hallucination.
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            To cheat the eye with blear illusions. --Milton.
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   2. Hence: Anything agreeably fascinating and charming;
      enchantment; witchery; glamour.
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            Ye soft illusions, dear deceits, arise! --Pope.
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   3. (Physiol.) A sensation originated by some external object,
      but so modified as in any way to lead to an erroneous
      perception; as when the rolling of a wagon is mistaken for
      thunder.
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   Note: Some modern writers distinguish between an illusion and
         hallucination, regarding the former as originating with
         some external object, and the latter as having no
         objective occasion whatever.
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   4. A plain, delicate lace, usually of silk, used for veils,
      scarfs, dresses, etc.

   Syn: Delusion; mockery; deception; chimera; fallacy. See
        Delusion. Illusion, Delusion. Illusion refers
        particularly to errors of the sense; delusion to false
        hopes or deceptions of the mind. An optical deception is
        an illusion; a false opinion is a delusion. --E.
        Edwards.
        [1913 Webster]

WordNet (r) 3.0 (2006):

illusion
    n 1: an erroneous mental representation [syn: illusion,
         semblance]
    2: something many people believe that is false; "they have the
       illusion that I am very wealthy" [syn: illusion, fantasy,
       phantasy, fancy]
    3: the act of deluding; deception by creating illusory ideas
       [syn: delusion, illusion, head game]
    4: an illusory feat; considered magical by naive observers [syn:
       magic trick, conjuring trick, trick, magic,
       legerdemain, conjuration, thaumaturgy, illusion,
       deception]