APA Dictionary of Psychology

a state of impaired mental functioning in which awareness is retained but with loss of cognitive coherence and orientation to time, place, and sometimes identity. It may be accompanied by rambling or incoherent speech, visual hallucinations, and psychomotor disturbances. It can arise from a wide variety of causes, including brain lesions, trauma, toxicity, medications, neurotransmitter imbalances, sleep disturbances, sedation, or fever. See also delirium.