An Etymological Dictionary of Astronomy and Astrophysics

1. A false sense impression of something actually present.
   

2. Something that produces a false or misleading impression of reality.
   → optical illusion.
   

3. An instance of being deceived.

Etymology (EN): 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.”

Etymology (PE): Safmân, literally “wrong thought, ~ idea,” from saf “wrong” (Tâleši), → false, + mân “thought,” → idea.