Fatigue (also called exhaustion, tiredness, lethargy, languidness, languor, lassitude, and listlessness) is a state of awareness describing a range of afflictions, usually associated with physical and/or mental weakness, though varying from a general state of lethargy to a specific work-induced burning sensation within one's muscles. Physical fatigue is the inability to continue functioning at the level of one's normal abilities. It is widespread in everyday life, but usually becomes particularly noticeable during heavy exercise. Mental fatigue, on the other hand, rather manifests in somnolence (sleepiness).
Fatigue is a non-specific symptom, which means that it has many possible causes. Fatigue is considered a symptom, as opposed to a medical sign, because it is reported by the patient instead of being observed by others. Fatigue and "feelings of fatigue" are often confused.