A pseudohallucination is an involuntary sensory experience vivid enough to be regarded as a hallucination

Hallucination

A hallucination, in the broadest sense of the word, is a perception in the absence of a stimulus. In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli which have qualities of real perception, in that they are vivid,...

, but recognised by the patient not to be the result of external stimuli. In other words, it is a hallucination that is recognized as a hallucination, as opposed to a "normal" hallucination which would be perceived as real. An example used in psychiatry is the hearing of voices which are inside the head according to the patient; in contrast, a hallucination would be indistinguishable to the patient from a real external stimulus, e.g. people were talking about me.

The term is not widely used in the psychiatric and medical fields, as it is considered ambiguous. The term "nonpsychotic hallucination" is more preferred. Pseudohallucinations, then, are more likely to happen with a hallucinogenic drug. Thus, when one speaks of hallucinating when under the influence of such a drug (excluding deliriants), they are probably referring to pseudohallucinations.

They are considered a feature of conversion disorder

Conversion disorder

Conversion disorder is a condition in which patients present with neurological symptoms such as numbness, blindness, paralysis, or fits without a neurological cause. It is thought that these problems arise in response to difficulties in the patient's life, and conversion is considered a psychiatric...

, somatization disorder

Somatization disorder

Somatization disorder is a psychiatric diagnosis applied to patients who persistently complain of varied physical symptoms that have no identifiable physical origin...

, and dissociative disorders.