An acute change in mental status that is characterized by a sudden onset of impaired attention, disorganized thinking, or incoherent speech. The patient usually has clouded consciousnesses, perceptual disturbances, sleep-wake problems, psychomotor agitation or lethargy and is disoriented.

Delirium is an important problem for all clinical services providing care for older people, particularly emergency  departments; general medical, elderly care, surgical, and oncology wards; intensive care units; and, in the community, residential and nursing homes. Delirium is the most common complication of hospital admission for older people.