In 1978 the American neuroanatomist Fred H. Johnson defined hallucinations as follows: "In essence, hallucinations are just subjective covert conversations, rather than objective overt talking, and they at times need not be seen as abnormal, or, for that matter, as of much interest to anyone."
References
Johnson, F.H. (1978). The anatomy of hallucinations. Chicago, IL: Nelson-Hall.
Dictionary of Hallucinations. J.D. Blom. 2010.