Apathy is a psychological term for a state of indifference — where an individual is unresponsive or "indifferent" to aspects of emotional, social, or physical life. Clinical apathy is considered to be at an elevated level, while a moderate level might be considered  depression, and an extreme level could be diagnosed as a dissociative disorder. The physical aspect of apathy associated with physical deterioration, muscle loss, and lack of energy is called lethargy — which has many  pathological causes as well.