A hallucination is perceiving anything that was not caused by an actual stimuli. Many drugs can cause hallucinations but they are only typical with hallucinogens. Hallucinogens are made up of psychedelics, dissociatives, cannabinoids, and deliriants. Hallucinations can occur without drugs in people with brain injuries or sever mental disorders.
Hallucinations can be experienced through any sense and not just visual or auditory. Visual hallucinations can be either open eye visuals (OEV) or closed eye visuals (CEV).