In 1868 the German psychiatrist Friedrich Wilhelm Hagen (1814-1888) defined hallucinations as "the lively manifestation of a subjectively arising image (among which also tones, words, feelings) in addition to - and simultaneous with - true perceptions, and in equal effect with these."
References
Hagen, F.W. (1868). Zur Theorie der Hallucina-tionen. Allgemeine Zeitschrift für Psychiatrie, 25, 1-113.
Dictionary of Hallucinations. J.D. Blom. 2010.