Chronic fatigue syndrome is a disorder characterised by persistent fatigue, and accompanied by other specific symptoms, for a minimum duration of six months.

Some of the major symptoms of CFS include post-exertion malaise, fatigue that is not related to exertion and is not substantially relieved by rest, muscle weakness, generalised muscle and joint aches, sensitivity to cold, tendency to overheat and sweat with mild exertion or in bed at night, sleep disturbances, unrefreshing sleep, severe mental exhaustion, headaches, light-headedness, shortness of breath, palpitations, digestive disturbances, food intolerances, frequent colds and sore throats, depression and anxiety.

The list of symptoms, and severity of those symptoms varies widely from person to person. While there has been some controversy surrounding the correct naming of this disorder, it is certain that it is significantly debilitating and quality of life is, often severely, impaired. The term chronic fatigue immune dysfunction syndrome or CFIDS, is used less frequently but is used as a synonym for chronic fatigue syndrome.