1. (noun)delusion, psychotic belief (psychology) an erroneous belief that is held in the face of evidence to the contrary
2. (noun)delusion, hallucination a mistaken or unfounded opinion or idea "he has delusions of competence"; "his dreams of vast wealth are a hallucination"
3. (noun)delusion, illusion, head game the act of deluding; deception by creating illusory ideas