Delirium: A sudden state of severe confusion and rapid changes in brain function, sometimes associated with hallucinations and hyperactivity, in which the patient is inaccessible to normal contact. Symptoms may include inability to concentrate and disorganized thinking evidenced by rambling, irrelevant, or incoherent speech. There may be a reduced level of consciousness, sensory misperceptions and illusions, disturbances of sleep, drowsiness, disorientation to time, place, or person, and problems with memory.

Delirium can be due to a number of conditions that derange brain metabolism, including infection, brain tumor, poisoning, drug toxicity or withdrawal, seizures, head trauma, and metabolic disturbances such as fluid, electrolyte, or acid-base imbalance, hypoxia, hypoglycemia, or hepatic or renal failure.

Also called acute confusional state and acute brain syndrome.