noun (n)

• illusory perception; a common symptom of severe mental disorder(noun.state)

  source: wordnet30
• a mistaken or unfounded opinion or idea(noun.cognition)

  Example:
  His dreams of vast wealth are a hallucination.
  

  Synonym:
  delusion
  

  source: wordnet30
• an object perceived during a hallucinatory episode(noun.cognition)

  Example:
  He refused to believe that the angel was a hallucination.
  

  source: wordnet30
• The act of hallucinating; a wandering of the mind; error; mistake; a blunder.(noun)

  source: webster1913