Fibromyalgia (FM), or fibromyalgia syndrome (FMS), as it is sometimes called, is a complex condition that causes persistent pain in muscles, ligaments, tendons, and joints. In addition to chronic widespread pain (experienced primarily as muscle aching or stiffness by 100% of individuals with FM), this syndrome may cause other symptoms, most often fatigue (reported by approximately 90% of people with FM), disturbed sleep, and headaches. Symptoms can be made worse or triggered by anxiety, cold or drafty environments, depression, hormonal changes before menstrual periods or during menopause, physical overexertion, stress, and weather changes.

According to a 1990 definition of the American College of Rheumatologists (ACR Link), a diagnosis of fibromyalgia includes:

1. Widespread pain in all four quadrants for at least 3 months. This includes some axial (cervical spine, anterior chest, thoracic spine, or low back) pain.
2. In addition, patients report abnormal pain sensitivity in at least 11 out of 18 possible tender points.

FM can exist by itself, but is usually accompanied by a primary or secondary chronic medical condition such as rheumatoid arthritis, hypothyroidism, and, most particularly, chronic fatigue syndrome. However, these other medical conditions are not the source of the widespread pain. Stress or lack of sleep can further increase the severity of FM symptoms. Recent research has strongly suggested the involvement of the central nervous system in the development of fibromyalgia, along with a suspected genetic predisposition.