WordReference Random House Learner's Dictionary of American English © 2017
aˈtom•ic ˈclock, n. [countable]
  • Timean extremely accurate clock that uses the frequency of atoms of certain substances, as cesium, to regulate its speed.

  • WordReference Random House Unabridged Dictionary of American English © 2017
    atomic clock, 
  • Timean extremely accurate electronic clock regulated by the resonance frequency of atoms or molecules of certain substances, as cesium.
    • 1935–40

    Collins Concise English Dictionary © HarperCollins Publishers::

    atomic clock n
    1. 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


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