noun

1. a false idea or conception; belief or opinion not in accord with the facts
2. an unreal, deceptive, or misleading appearance or image a large mirror giving the illusion of space in a small room
3. 1. a false perception, conception, or interpretation of what one sees, where one is, etc.
   2. the misleading image resulting in such a false impression
4. hallucination
5. a delicate, gauzy silk tulle used for veils, etc.

Etymology: ME illusioun < OFr illusion < L illusio, a mocking (in LL(Ec), deceit, illusion) < illusus, pp. of illudere, to mock, play with < in-, on + ludere, to play: see ludicrous