Real Dictionary

atomic clock in American English
an extremely accurate clock whose precision depends upon the very constant frequency at which atoms (or molecules) of certain substances, as of cesium, rubidium, or ammonia, absorb or emit electromagnetic radiation
noun: an extremely accurate electronic clock regulated by the resonance frequency of atoms or molecules of certain substances, as cesium

atomic clock in British English
noun: an extremely accurate clock in which an electrical oscillator is controlled by the natural vibrations of an atomic or molecular system such as caesium or ammonia