Fatigue syndrome, also known as chronic fatigue syndrome (CFS), is a complex disorder characterized by extreme fatigue that doesn't improve with rest and lasts for more than six months. There are various synonyms for this condition, including myalgic encephalomyelitis (ME), post-viral fatigue syndrome (PVFS), systemic exertion intolerance disease (SEID), and chronic fatigue immune dysfunction syndrome (CFIDS). ME is a term mostly used in Europe, while PVFS and CFIDS are more commonly used in North America. SEID is a relatively new term, proposed by the Institute of Medicine in 2015, to describe the condition more accurately. No matter what it's called, fatigue syndrome can significantly impact a person's quality of life and require long-term management.