The label covers a set of symptoms (Pain, tiredness, depression, stiffness, and lack of rest from sleep), united only by being idiopathic. (Without a known cause).

From a diagnostic standpoint fibromylagia is throwing your hands in the air and going "Something's fucky, but we don't know what it is". Which doesn't mean that the person doesn't suffer, some people with this diagnosis are basically bedridden and in agony (Others have mild aches and pains, which still sucks)