Illusion
Part of speech: noun
Definition: Anything that seems to be something that it is not. A misapprehension; a belief in something that is in fact not true. A magician's trick. The fact of being an illusion (in any of the above senses).
Hallucination
Part of speech: noun
Definition: A sensory perception of something that does not exist, arising from disorder of the nervous system, as in delirium tremens; a delusion.The act of hallucinating; a wandering of the mind; an error, mistake or blunder.