Hallucination is a general symptom where there is 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). [ url:http://www2.merriam-webster.com/cgi-bin/mwmednlm?book=Medical&va=hallucinations ]