delusion
n. C / Un. a false belief that you keep even when there is clear proof that it is not true.
n. a persistent false belief maintained despite strong contradictory evidence, often as a symptom of mental disorder or extreme self-deception.
He is under the delusion that he can win without practicing.
The patient suffered from the delusion that he was being followed by secret agents despite having no evidence.
The company's leadership operated under the collective delusion that their market share was untouchable, ignoring the rapid technological shifts that eventually led to their bankruptcy.
From Latin dēlūsiō.
Often used with the preposition 'under' ('under the delusion that') or 'of' ('delusions of grandeur').
he has the illusion that he is richhe has the delusion that he is richWhile 'illusion' refers to a deceptive appearance or trick of the senses, 'delusion' refers specifically to a fixed, false belief.