Hallucination, n.
Etymology: [L. hallucinatio cf. F. hallucination.]
1. The act of hallucinating; a wandering of the mind; error; mistake; a blunder. This must have been the hallucination of the transcriber. Addison.
2. (Med.)
The perception of objects which have no reality, or of sensations which have no corresponding external cause, arising from disorder or the nervous system, as in delirium tremens; delusion. Hallucinations are always evidence of cerebral derangement and are common phenomena of insanity. W. A. Hammond.
Shades of meaning
Fernald has no entry of its own for hallucination; he lists it under delusion, dream, insanity.
Where he writes about it:
A hallucination is a false image or belief which has nothing, outside of the disordered mind, to suggest it; as, the hallucinations of delirium tremens. — from delusion
Hallucination is the seeming perception of non-existent objects, as in insanity or delirium. — from dream
Hallucination is the apparent perception of that which does not exist or is not present to the senses, as the seeing of specters or of reptiles in delirium tremens. — from insanity