How do you sign “illusion” in sign language?

I could not find illusion but maybe this will help you. It is where I go when I need to find a word.

http://commtechlab.msu.edu/sites/aslweb/browser.htm

Also here is some synonyms for illusion
apparition, bubble*, chimera, confusion, daydream, deception, déjè vu, delusion, error, fallacy, false impression, fancy, fantasy, figment of imagination, fool’s paradise, ghost, hallucination, head trip, hocus-pocus*, idolism, ignus fatuus, image, invention, make-believe, mirage, misapprehension, misbelief, misconception, misimpression, mockery, myth, optical illusion, paramnesia, phantasm, pipe dream*, rainbow*, seeming, semblance

these are according to this web site

http://thesaurus.reference.com/

hope I helped!