The 1982 Manual for the Assessment and Documentation of Psychopathology (AMDP) defines hallucinations as follows: "Hallucinations are perceptual experiences without a corresponding stimulus in the environment. One can hallucinate in all sense modalities and frequently in more than one. The judgment of reality is more or less narrowed or suspended."
References
Guy, W., Ban, T.A., eds. (1982). The AMDP-system: Manual for the assessment and documentation of Psychopathology. Berlin: Springer.
Dictionary of Hallucinations. J.D. Blom. 2010.