pseudohallucination

from The Century Dictionary.

• noun In mental pathol., an apperceptive or psychic hallucination; a hallucination which results from the exercise of memory and imagination without the interposition of a sensory stimulus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A brief but vivid visual or auditory experience that is recognised as being unreal

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an image vivid enough to be a hallucination but recognized as unreal