As with age, there is not a generally valid definition for aging.

Aging is the sum of all signs of wear and tear during the lifetime. (Selye  1962, 4).

Aging is the result of changes that individuals show in a characteristic way with the advance of time, from adulthood to the end of life (Singer 1981, 19).

With regard to sport, aging could be described as the sum of all biological, psychological, and social changes which lead to the gradual reduction of psychophysical adaptation and capability of the individual after reaching adulthood and passing the age of peak performance (c.f. Weineck, Sportbiologie, 2000).

Physiological aging = synchronous changes to all organs and tissues

Pathological aging = marked emergence of the deficiency of an organ or system (Joblanovskij 1953, 540).