• synonyms


See more synonyms on Thesaurus.com
  1. having false or unrealistic beliefs or opinions: Senators who think they will get agreement on a comprehensive tax bill are delusional.
  2. 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.
Sometimes de·lu·sion·ar·y.


[dih-loo-zhuh n]
  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 of delusion

1375–1425; late Middle English < Latin dēlūsiōn- (stem of dēlūsiō), equivalent to dēlūs(us) (past participle of dēlūdere; see delude) + -iōn- -ion
Related formsde·lu·sion·al, de·lu·sion·ar·y, adjectivepre·de·lu·sion, noun
Can be confusedallusion delusion elusion hallucination illusion (see synonym study at illusion)


See more synonyms on Thesaurus.com
1. deception. See illusion.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for delusionary


  1. a mistaken or misleading opinion, idea, belief, etche has delusions of grandeur
  2. psychiatry a belief held in the face of evidence to the contrary, that is resistant to all reasonSee also illusion, hallucination
  3. the act of deluding or state of being deluded
Derived Formsdelusional, adjectivedelusive, adjectivedelusively, adverbdelusiveness, noundelusory (dɪˈluːsərɪ), adjective
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for delusionary



1871, from delusion + -al (1).



"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.

Online Etymology Dictionary, © 2010 Douglas Harper

delusionary in Medicine


  1. A false belief strongly held in spite of invalidating evidence, especially as a symptom of mental illness.
Related formsde•lusion•al adj.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

delusionary in Science


  1. A false belief or perception strongly held in spite of invalidating evidence, especially as a symptom of mental illness, as in schizophrenia.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

delusionary in Culture


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 New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.