delusion . fact or fiction.
A false belief or opinion: labored under the delusion that success was at hand. b. Psychiatry A false belief or perception that is a manifestation of a mental illness: delusions of persecution. [Middle English delusioun, from Latin dēlūsiō, dēlūsiōn-, from dēlūsus, past participle of dēlūdere, to delude; see delude.]