1) A false sense impression of something actually present.
From O.Fr. illusion "a mocking," from L. illusionem (nom. illusio) "a mocking, irony," from illudere "to mock at," literally "to play with," from in- "at" + ludere "to play," from ludus "a game, play;" PIE base *leid- "to play, jest."
Fr.: illusion d'optique
A perception of visual stimuli in which what is perceived is in a way different from the way it is in reality. Same as visual illusion.