Pseudohallucination
A pseudohallucination is a sensory experience vivid enough to be regarded as a hallucination, but recognised not to be the result of external stimuli. An example used in psychiatry is the hearing of voices which are inside the head according to the patient; in contrast, an 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.[1] Pseudohallucinations, then, are more likely to happen with an hallucinogenic drug. Thus, when one speaks of hallucinating when under the influence of such a drug (excluding deliriants), they are probably referring to pseudohallucinations.