de·lu·sion 
(dĭ-lzhən)
(dĭ-lzhən)n.
1.
a. The act or process of deluding.
b. The state of being deluded.
2.
a. 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.]
de·lusion·al adj.