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"act of misleading someone," early 15c.; as a form of mental derangement, 1550s, from Latin delusionem (nominative delusio) "a deceiving," noun of action from past participle stem of deludere (see delude).
Technically, delusion is a belief that, though false, has been surrendered to and accepted by the whole mind as a truth; illusion is an impression that, though false, is entertained provisionally on the recommendation of the senses or the imagination, but awaits full acceptance and may not influence action. Delusions of grandeur, the exact phrase, is recorded from 1840, though the two words were in close association for some time before that.
delusion de·lu·sion (dĭ-lōō'zhən)
A false belief strongly held in spite of invalidating evidence, especially as a symptom of mental illness.
in psychology, a rigid system of beliefs with which a person is preoccupied and to which the person firmly holds, despite the logical absurdity of the beliefs and a lack of supporting evidence. Delusions are symptomatic of such mental disorders as paranoia, schizophrenia, and major depression and of such physiological conditions as senile psychosis and delirium. They vary in intensity, extent, and coherence and may represent pathological exaggeration of normal tendencies to rationalization, wishful thinking, and the like. Among the most common are delusions of persecution and grandeur; others include delusions of bodily functioning, guilt, love, and control