In 1907 the German psychiatrists Otto Bin-swanger (1852-1929) and Ernst Siemerling (1857-1931) defined hallucinations as follows. "We designate hallucinations as sense perceptions which, without stimulation of the sense organ involved by an external object, occur as a mere consequence of internal stimuli."
References
Binswanger, O., Siemerling, E. (1907). Lehrbuch der Psychiatrie. Zweite vermehrte Auflage. Jena: Verlag von Gustav Fischer.
Dictionary of Hallucinations. J.D. Blom. 2010.