Fatigue (physiology), exhaustion of mind or body resulting from labor or exertion. Certain diseases, such as diabetes, cancer, and heart ailments, can also cause fatigue. Fatigue is characterized by the inability to perform physical tasks at one's usual pace or strength, and by a slowing of the thinking processes that may involve failure of memory.

The term fatigue is also used to denote a decreased susceptibility of a sensory receptor, such as the eye, to sensation. A red surface seems to become gray when it is stared at continuously, and the retina is said to be fatigued by long exposure to the color red. The same is true of hearing. A prolonged noise that is heard continuously becomes less intense due to fatigue of the processes of the inner ear. Continued mental exertion, as in a protracted working over of a problem, causes the condition known as mental fatigue.