In 2004 the British neurologist and psychiatrist Anthony S. David (b. 1958) defined hallucinations as follows. "A sensory experience which occurs in the absence of corresponding external stimulation of the relevant sensory organ, has a sufficient sense of reality to resemble a veridical perception, over which the subject does not feel s/he has direct and voluntary control, and which occurs in the awake state."
References
David, A.S. (2004). The cognitive neuropsy-chiatry of auditory verbal hallucinations: An overview. Cognitive Neuropsychiatry,9, 107-123.
Dictionary of Hallucinations. J.D. Blom. 2010.