Delirium, a disturbance of the mind in which a person is unable to recognize the surroundings. It may be accompanied by restlessness, incoherence, excitement, or hallucination. Delirium is not a disease, but a symptom. It may occur in the course of a nervous or mental disease, or as a result of other types of illness, fever, or injury (particularly a head injury). Delirious persons should be constantly observed to prevent them from hurting themselves.