n 1  :  a misleading image presented as a visual stimulus  2 a  :  perception of something objectively existing in such a way as to cause misinterpretation of its actual nature : esp  :  OPTICAL ILLUSION - compare DELUSION2    b  :  HALLUCINATION 1  c  :  a pattern capable of reversible perspective il*lu*sion*al adj 
 
 
 
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