In 1972 the Canadian psychologist Graham F. Reed (1923-1989) defined illusions as follows: "Illusion involves the misinterpretation of input in terms of its synthesis with stored material."
References
Reed, G. (1972). The psychology ofanomalous experience. A cognitive approach. London: Hutchinson & Co.
Dictionary of Hallucinations. J.D. Blom. 2010.