delusion

American  

[dih-loo-zhuhn] / dɪˈlu ʒən /

noun

1. an act or instance of deluding.

   Synonyms:

   deception

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.

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delusion British  

/ dɪˈluːʒən, dɪˈluːsərɪ /

noun

1. a mistaken or misleading opinion, idea, belief, etc

   he has delusions of grandeur

2. psychiatry a belief held in the face of evidence to the contrary, that is resistant to all reason See also illusion hallucination

3. the act of deluding or state of being deluded

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delusion Scientific  

/ dĭ-lo̅o̅′zhən /

1. A false belief or perception strongly held in spite of invalidating evidence, especially as a symptom of mental illness, as in schizophrenia.

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delusion Cultural  

1. 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.

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Usage

What is a delusion? A delusion is a false belief or opinion, especially one held in resistance to strong evidence against it, as in Even after losing five straight championships, Heather still had the delusion that she was the best in the world. In psychiatry, delusion is used to mean an unshakeable belief in something that isn’t true. They believe it because they have a mental disorder, such as schizophrenia. More generally, delusion can be used to mean an act of deluding, as in The crowd didn’t fall for the swindler’s attempts at delusion. Delusion can also refer to the state of being deluded, as in The emperor’s delusion was the work of the evil, scheming vizier. Delusion is similar to the words illusion and hallucination, which also describe false beliefs or experiences. An illusion is a false image that is a result of a distortion or manipulation of actual things. For example, makeup can create the illusion that a person is younger than they actually are. A hallucination is a false sensory experience that isn’t rooted in reality at all. On the other hand, a delusion is often somewhat based on reality but a person’s belief is inaccurate due to wrong information or their own hubris. Example: Despite his low test scores, Gerard strongly believed the delusion that he was the smartest kid in the class.

Synonym Usage

See illusion.

Other Word Forms

Derived Forms

• delusively
  adverb
• delusory
  adjective
• delusive
  adjective
• delusionary
  adjective
• predelusion
  noun
• delusional
  adjective
• delusiveness
  noun

Etymology

Origin of delusion

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

Explanation

A delusion is a belief that has no evidence in fact — a complete illusion. The cook at the hot dog stand who thinks he is the best chef in the world? That opinion is definitely a delusion. The noun delusion is often used in the phrase delusions of grandeur, which expresses the belief that unattainable goals are well within reach, like a terrible actress's delusions of grandeur that she won't just land her first role in a movie, it also will make her an Academy Award winner. Delusions like that can be amusing for onlookers, but other kinds of delusions are not, like those suffered by the mentally ill. Their delusions can remove them from reality, making it hard to function.

Example Sentences

John Carney directs a musical dramedy starring the actor and Nick Jonas in a story of ambition, delusion and betrayal.

From The Wall Street Journal • May 28, 2026

"Any delusion of full decoupling isn't realistic on any near-term horizon", she told AFP.

From Barron's • May 13, 2026

They concluded that the chatbots appropriately identified problematic prompts, like a feeling of being watched by phantom people or feeling personally persecuted by the government, as a developing delusion only about half the time.

From Slate • Feb. 2, 2026

A U.K. police chief is under fire after admitting a delusion from Microsoft Copilot produced incorrect evidence.

From MarketWatch • Jan. 14, 2026

It was a delusion that would put me on a collision course with reality at the US Secret Classic meet the following July.

From "Courage to Soar" by Simone Biles

Related Words

• deception
• fantasy
• hallucination
• illusion
• pipe dream