hallucination - definition and synonyms

noun 

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/həˌluːsɪˈneɪʃ(ə)n/
singularhallucination
pluralhallucinations
  1. [countable] something that you think you can see or hear that is not really there, especially because of an illness or the effect of drugs
    1. a.
      [uncountable] the experience of seeing or hearing something that is not really there