CFS/ME, also called chronic fatigue syndrome or myalgic encephalopathy, is a condition characterized by an abnormal and persistent feeling of exhaustion and fatigue lasting for more than six months, significantly limiting daily life. The term "syndrome" refers to a collection of different symptoms that occur in a specific pattern.

In Denmark, the Health Authority recommends using the diagnosis "functional disorder" instead of CFS/ME. The diagnosis is based on the patient’s description of symptoms and by excluding other diseases, as there are no tests, blood samples, or scans that can confirm the condition.

The main symptom is pronounced fatigue or exhaustion that does not go away even with rest or sleep. Many experience that even small physical or mental efforts can worsen the exhaustion. In addition, many have memory problems, difficulty concentrating, headaches, and dizziness, and some experience muscle and joint pain, digestive problems, or sleep disturbances. To be diagnosed, the fatigue must have lasted at least six months and cannot be better explained by another physical or mental illness.

The exact cause of CFS/ME is not known, but the condition is considered multifactorial, meaning that biological, psychological, cultural, and social factors interact. A distinction is often made between underlying, triggering, and maintaining factors, all of which can influence how the condition develops.