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.
      [1913 Webster]

            To cheat the eye with blear illusions. --Milton.
      [1913 Webster]

   2. Hence: Anything agreeably fascinating and charming;
      enchantment; witchery; glamour.
      [1913 Webster]

            Ye soft illusions, dear deceits, arise! --Pope.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "illusion":
   aberrancy, aberration, air, airy nothing, apparition, appearance,
   bamboozlement, bedevilment, befooling, bewitchery, bewitchment,
   bluffing, brainchild, bubble, calculated deception, captivation,
   chimera, circumvention, conning, deceiving, deception,
   deceptiveness, defectiveness, defrauding, delirium, delusion,
   delusiveness, deviancy, distortion, dream, dupery, eidolon,
   enchantment, enmeshment, ensnarement, entanglement, entrancement,
   entrapment, errancy, erroneousness, error, ether, fallaciousness,
   fallacy, falseness, falsity, fancy, fantasque, fantasy,
   fascination, fault, faultiness, fiction, figment, flaw, flawedness,
   flimflam, flimflammery, fond illusion, fooling, hallucination,
   hamartia, heresy, heterodoxy, hoodwinking, idle fancy,
   ignis fatuus, imagery, imagination, imagining, insubstantial image,
   invention, kidding, maggot, make-believe, maya, mirage,
   misapplication, misapprehension, misconception, misconstruction,
   misdoing, misfeasance, misinterpretation, misjudgment, mist,
   mistake, myth, obsession, outwitting, overreaching, peccancy,
   perversion, phantasm, phantasmagoria, phantom, pipe, pipe dream,
   possession, putting on, rainbow, romance, seeming,
   self-contradiction, self-deception, semblance, shadow, sick fancy,
   sin, sinfulness, smoke, snow job, song and dance, spirit, spoofery,
   spoofing, subterfuge, swindling, thick-coming fancies, thin air,
   trickiness, tricking, trip, unorthodoxy, untrueness, untruth,
   untruthfulness, vapor, victimization, vision, whim, whimsy,
   wildest dreams, willful misconception, wishful thinking, witchery,
   wrong, wrongness