de·lu·sion (d-lzhn) n. a. The act or process of deluding. b. The state of being deluded. 2. A false belief or opinion: labored under the delusion that success was at hand.3. Psychiatry A false belief strongly held in spite of invalidating evidence, especially as a symptom of mental illness: delusions of persecution. [Middle English delusioun, from Latin dlsi, dlsin-, from dlsus, past participle of dldere, to delude; see delude.]de·lusion·al adj.