In 1975 the American anthropologist Raoul Weston La Barre defined "illusions as follows: "'Illusion' is a false mental appearance made by some actual external cause acting on the senses but capable of conceptual correction. Thus mistaking a tree in the dark for a man is an illusion, of which the tree may be disabused by various forms of reality testing; emphasis is on the ready correctibility of an illusion."
References
La Barre, W. (1975). Anthropological perspectives on hallucination and hallucinogens.In: Hallucinations. Behavior, experience, and theory. Edited by Siegel, R.K., West, L.J. New York, NY: John Wiley & Sons.
Dictionary of Hallucinations. J.D. Blom. 2010.