A false belief held despite strong evidence against it; self-deception. Delusions are common in some forms of psychosis. Because of his delusions, the literary character Don Quixote attacks a windmill, thinking it is a giant.
delusion de·lu·sion (dĭ-l&oomacr;'zhən)
n.
A false belief strongly held in spite of invalidating evidence, especially as a symptom of mental illness.