In his classic textbook of psychiatry, the Swiss psychiatrist Paul Eugen Bleuler (1857-1939) defines hallucinations as follows. "Hallucinations are perceptions without a corresponding stimulus externally: everything that one perceives can also become a hallucination, in such a way, that the various elements can be combined freely; a hallucinated lion can have wings, a human figure can be composed out of the properties of various persons. Moreover, pathological brain functions evidently evoke internal perceptions that were not available before."
References
Bleuler, E. (1923). Lehrbuch der Psychiatrie. Vierte Auflage. Berlin: Verlag von Julius Springer.
Dictionary of Hallucinations. J.D. Blom. 2010.