Hallucinations true UMLS:C0018524 Hallucinations HP:0000738 Perceptions in a conscious and awake state that, in the absence of external stimuli, have qualities of real perception. These perceptions are vivid, substantial, and located in external objective space. Hallucination SNOMEDCT_US:7011001 UMLS:C0235153 Sensory hallucination Abnormality of mental function