dɪˈl(j)uːʒ(ə)n həˌluːsɪˈneɪʃən ɪˈl(j)uːʒ(ə)n the act of deluding; deception by creating illusory ideas She has a delusion that she is being spied on. a mistaken or unfounded opinion or idea he has delusions of competence his dreams of vast wealth are a hallucination (psychology) an erroneous belief that is held in the face of evidence to the contrary