An out-of-date term that was used to describe an ill-defined state of nervous exhaustion in which, although the patient suffers from no definite disease, he or she becomes incapable of sustained exertion. The condition which it represented is now believed to have been in many sufferers a form of NEUROSIS or psychosomatic disease. It was also used in the past to describe what is now called CHRONIC FATIGUE SYNDROME (CFS).