hallucination

Short definition from Wiktionary

Noun

1. 1
   A sensory perception of something that does not exist, often arising from disorder of the nervous system, as in delirium tremens.
2. 2
   The act of hallucinating; a wandering of the mind; an error, mistake or blunder.
3. 3
   A confident but incorrect response given by an artificial intelligence; a confabulation.