1. something that deceives by producing a false or misleading impression of reality.
  2. the state or condition of being deceived; misapprehension.
  3. an instance of being deceived.
  4. Psychology. a perception, as of visual stimuli (optical illusion), that represents what is perceived in a way different from the way it is in reality.
  5. a very thin, delicate tulle of silk or nylon having a cobwebbed appearance, for trimmings, veilings, and the like.
  6. Obsolete. the act of deceiving; deception; delusion.


  1. a false appearance or deceptive impression of realitythe mirror gives an illusion of depth
  2. a false or misleading perception or belief; delusionhe has the illusion that he is really clever
  3. psychol a perception that is not true to reality, having been altered subjectively in some way in the mind of the perceiverSee also hallucination
  4. a very fine gauze or tulle used for trimmings, veils, etc

mid-14c., “act of deception,” from Old French illusion “a mocking, deceit, deception” (12c.), from Latin illusionem (nominative illusio) “a mocking, jesting, irony,” from illudere “mock at,” literally “to play with,” from assimilated form of in- “at, upon” (see in- (2)) + ludere “to play” (see ludicrous). Sense of “deceptive appearance” developed in Church Latin and was attested in English by late 14c. Related: Illusioned “full of illusions” (1920).


  1. An erroneous perception of reality.
  2. An erroneous concept or belief.
  3. The condition of being deceived by a false perception or belief.
  4. Something, such as a fantastic plan or desire, that causes an erroneous belief or perception.
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