In 1990 the American psychiatrist Ghazi Asaad defined hallucinations as follows: "Hallucinations are generally defined as perceptions that occur in the absence of corresponding external stimuli. [...] True hallucinations are experienced and accepted by the patient as real perceptions. It is obvious that reality testing must be impaired in order for this symptom to appear."
References
Asaad, G. (1990). Hallucinations in clinical psychiatry. A guide for mental health professionals. New York, NY: Brunner/Mazel.
Dictionary of Hallucinations. J.D. Blom. 2010.