having false or unrealistic beliefs or opinions: Senators who think they will get agreement on a comprehensive tax bill are delusional.
Psychiatry. maintaining fixed false beliefs even when confronted with facts, usually as a result of mental illness: He was so delusional and paranoid that he thought everybody was conspiring against him.
"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.
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 characterDon Quixote attacks a windmill, thinking it is a giant.