1) Subjectively experienced sensations in the absence of an appropriate stimulus, but which are regarded by the individual as real. They may be of organic origin or associated with mental disorders.

2) Perceptions through any sense modality in the absence of an appropriate stimulus. (Usually indicative of abnormality but may be experienced occasionally by normal persons).

3) A hallucination is a false sensory perception in the absence of an external stimulus, as distinct from an illusion which is a misperception of an external stimulus. (from Wikipedia)

4) A perception of something (as a visual image or a sound) with no external cause usually arising from a disorder of the nervous system (as in delirium tremens or in functional psychosis without known neurological disease) or in response to drugs (as LSD).

5) The object of a hallucinatory perception.