noun Date: 1955 an inherent timing mechanism in a living system that is inferred to exist in order to explain the timing or periodicity of various behaviors and physiological states and processes (as fertility and aging)
BIOLOGICAL CLOCK
Meaning of BIOLOGICAL CLOCK in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012