Source : Webster's Revised Unabridged Dictionary (1913)

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.

            To cheat the eye with blear illusions. --Milton.

   2. Hence: Anything agreeably fascinating and charning;
      enchantment; witchery; glamour.

            Ye soft illusions, dear deceits, arise! --Pope.

   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.

   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.

   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.

Source : WordNet®

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