• A false or irrational belief held by a person despite evidence to the contrary.
  Harvard Dictionary of Health Terms
  Harvard Medical Publishing, 2011
• False belief regarding the self or persons or objects outside the self that persists despite the facts, and is not considered tenable by one's associates.
  CRISP Thesaurus
  National Institutes of Health, 2006
• False belief, based on incorrect inference about external reality, that is firmly held despite objective and obvious contradictory proof or evidence and despite the fact that other members of the culture do not share the belief.
  Sadock's Comprehensive Glossary of Psychiatry and Psychology
  BJ Sadock Titles Press, 2012
• False personal beliefs held contrary to reality, despite contradictory evidence and common sense.
  NCI Thesaurus
  U.S. National Cancer Institute, 2021
• Firmly held, illogical belief that is unsupported by evidence, and not shared by a sub-cultural group, with which a person is preoccupied.
  NICHD Pediatric Terminology
  U.S. National Cancer Institute, 2021
• A false belief regarding the self or persons or objects outside the self that persists despite the facts, and is not considered tenable by one's associates.
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2021
• A false belief that is held despite evidence to the contrary.
  Human Phenotype Ontology (HPO)
  The Human Phenotype Ontology Project, 2021
• A disorder characterized by false personal beliefs held contrary to reality, despite contradictory evidence and common sense.
  Common Terminology Criteria for Adverse Events
  U.S. National Institutes of Health, 2021
• 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...
  Encyclopedia Britannica
  Encyclopedia Britannica, Inc., 2020