• A false sensory perception in the absence of an external stimulus, as distinct from an illusion which is a misperception of an external stimulus.
  NCI
  U.S. National Cancer Institute, 2021
• A perception of something that is not really there.
  Harvard Dictionary of Health Terms
  Harvard Medical Publishing, 2011
• A sight, sound, smell, taste, or touch that a person believes to be real but is not real. Hallucinations can be caused by nervous system disease, certain drugs, or mental disorders.
  NCI Dictionary of Cancer Terms
  U.S. National Cancer Institute, 2021
• False sensory perception occurring in the absence of any relevant external stimulation of the sensory modality involved. For types of hallucinations, see the specific term.
  Sadock's Comprehensive Glossary of Psychiatry and Psychology
  BJ Sadock Titles Press, 2012
• Perception of an object or event without an external source or stimulus; which is perceived as real.
  NICHD Pediatric Terminology
  U.S. National Cancer Institute, 2021
• Subjectively experienced sensations in the absence of an appropriate stimulus, but which are regarded by the individual as real; may be of organic origin, drug induced, or associated with a mental disorder.
  CRISP Thesaurus
  National Institutes of Health, 2006
• Perceptions in a conscious and awake state that, in the absence of external stimuli, have qualities of real perception. These perceptions are vivid, substantial, and located in external objective space.
  Human Phenotype Ontology (HPO)
  The Human Phenotype Ontology Project, 2025
• A disorder characterized by a false sensory perception in the absence of an external stimulus.
  Common Terminology Criteria for Adverse Events
  U.S. National Institutes of Health, 2021
• Hallucination, the experience of perceiving objects or events that do not have an external source, such as hearing one's name called by a voice that no one else seems to hear. A hallucination is distinguished from an illusion, which is a misinterpretation of an actual stimulus. A historical survey...
  Encyclopedia Britannica
  Encyclopedia Britannica, Inc., 2025