Chronic fatigue syndrome (also known as myalgic encephalomyelitis) is a disorder of indeterminate etiology characterized by impairment of daily function due to onset of persistent, often profound, recurring fatigue not caused by overexertion, not relieved by rest, and lasting at least 6 months. Additional diagnostic criteria include postexertional malaise and lack of refreshment by sleep. Other signs and symptoms are related to cognitive function (ie, difficulty concentrating, memory impairment) and orthostatic intolerance, including headache, muscle and joint aches, disturbed sleep, sore throat, and tender lymph nodes.

The US National Academy of Medicine has proposed the new name of systemic exertion intolerance disease.