delusion

[dih-loo-zhuhn]
noun
1.
an act or instance of deluding.
2.
the state of being deluded.
3.
a false belief or opinion: delusions of grandeur.
4.
Psychiatry. a fixed false belief that is resistant to reason or confrontation with actual fact: a paranoid delusion.

Origin:
1375–1425; late Middle English < Latin dēlūsiōn- (stem of dēlūsiō), equivalent to dēlūs(us) (past participle of dēlūdere; see delude) + -iōn- -ion

delusional, delusionary, adjective
predelusion, noun

allusion, delusion, elusion, hallucination, illusion (see synonym study at illusion).


1. deception. See illusion.
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World English Dictionary
delusion (dɪˈluːʒən)
 
n
1.  a mistaken or misleading opinion, idea, belief, etc: he has delusions of grandeur
2.  psychiatry illusion See also hallucination a belief held in the face of evidence to the contrary, that is resistant to all reason
3.  the act of deluding or state of being deluded
 
de'lusional
 
adj
 
de'lusive
 
adj
 
de'lusively
 
adv
 
de'lusiveness
 
n
 
delusory
 
adj

Collins English Dictionary - Complete & Unabridged 10th Edition
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Etymonline
Word Origin & History

delusion
"act of misleading someone," early 15c.; as a form of mental derangement, 1550s. 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.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

delusion de·lu·sion (dĭ-lōō'zhən)
n.
A false belief strongly held in spite of invalidating evidence, especially as a symptom of mental illness.


de·lu'sion·al adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
delusion   (dĭ-l'zhən)  Pronunciation Key 
A false belief strongly held in spite of invalidating evidence, especially as a symptom of mental illness, as in schizophrenia.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

delusion definition


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.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

delusion

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

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Here are five of science history's most bizarre hoaxes and delusions.
The psychiatrist tells her that her husband's delusion is harmless.
Or maybe it is just words of delusion, like a teenager who vows to stay out of
  trouble but knows he never will.
It was part denial, part delusion.
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