In 1913 the German psychiatrist and philosopher Karl Jaspers (1883-1969) defined illusions and hallucinations as follows: "Illusion is the term for perceptions which in fact are transpositions (or distortions) of real perceptions; here external sensory stimuli unite with certain transposing (or distorting) elements so that in the end we cannot differentiate the one from the other. Hallucinations are perceptions that spring into being in a primary way and are not transpositions or distortions, ofany genuine perception."
References
Jaspers, K. (1997). Generalpsychopathology. Volume 1. Translated by Hoenig, J., Hamilton, M.W. Baltimore, MA: Johns Hopkins University Press.
Dictionary of Hallucinations. J.D. Blom. 2010.