In 1935 the Austrian-American child psychiatrist Leo Kanner (1894-1981) defined hallucinations as follows: "Hallucinations are sensory impressions without external stimulation. There is no outward source for the imagined voices, visions, odors, tastes, or tactile sensations."
References
Kanner, L. (1935). Child psychiatry. Springfield, IL: Charles C. Thomas.
Dictionary of Hallucinations. J.D. Blom. 2010.