noun Date: 1938 a precision clock that depends for its operation on an electrical oscillator regulated by the natural vibration frequencies of an atomic system (as a beam of cesium atoms)
ATOMIC CLOCK
Meaning of ATOMIC CLOCK in English
Merriam Webster. Explanatory English dictionary Merriam Webster. Толковый словарь английского языка Мерриам-Уэбстер. 2012