Merriam-Webster Logo
  • Dictionary
  • Thesaurus
  • Scrabble
  • Spanish Central
  • Learner's Dictionary


noun de·lu·sion \di-ˈlü-zhən, dē-\

Simple Definition of delusion

  • : a belief that is not true : a false idea

  • : a false idea or belief that is caused by mental illness

Source: Merriam-Webster's Learner's Dictionary

Full Definition of delusion

  1. 1 :  the act of deluding :  the state of being deluded

  2. 2 a :  something that is falsely or delusively believed or propagated b :  a persistent false psychotic belief regarding the self or persons or objects outside the self that is maintained despite indisputable evidence to the contrary; also :  the abnormal state marked by such beliefs

delusional play \-ˈlüzh-nəl, -ˈlü-zhə-nəl\ adjective
delusionary play \-zhə-ˌner-ē\ adjective

Examples of delusion in a sentence

  1. He has delusions about how much money he can make at that job.

  2. He is living under the delusion that he is incapable of making mistakes.

  3. She is under the delusion that we will finish on time.

  4. As the illness progressed, his delusions took over and he had violent outbursts.

Origin of delusion

Middle English, from Late Latin delusion-, delusio, from deludere

First Known Use: 15th century

Synonym Discussion of delusion

delusion, illusion, hallucination, mirage mean something that is believed to be true or real but that is actually false or unreal. delusion implies an inability to distinguish between what is real and what only seems to be real, often as the result of a disordered state of mind <delusions of persecution>. illusion implies a false ascribing of reality based on what one sees or imagines <an illusion of safety>. hallucination implies impressions that are the product of disordered senses, as because of mental illness or drugs <suffered from terrifying hallucinations>. mirage in its extended sense applies to an illusory vision, dream, hope, or aim <claimed a balanced budget is a mirage>.

DELUSION Defined for Kids


noun de·lu·sion \di-ˈlü-zhən\

Definition of delusion for Students

  1. :  a false belief that continues in spite of the facts <“You might be laboring under the delusion that the entire … world is impressed with you …” — J. K. Rowling, Goblet of Fire>

Medical Dictionary


noun de·lu·sion \di-ˈlü-zhən\

Medical Definition of delusion

  1. 1a:  the act of deluding :  the state of being deludedb:  an abnormal mental state characterized by the occurrence of psychotic delusions

  2. 2:  a false belief regarding the self or persons or objects outside the self that persists despite the facts and occurs in some psychotic states—compare hallucination 1, illusion 2a

Seen and Heard

What made you want to look up delusion? Please tell us where you read or heard it (including the quote, if possible).


the range of perception or understanding

Get Word of the Day daily email!


Take a 3-minute break and test your skills!


Which of these is a synonym of nonplus?

reduce perplex soothe disapprove
Name That Thing

Test your visual vocabulary with our 10-question challenge!


Test Your Knowledge - and learn some interesting things along the way.