atomic clock

/AH0 T AA1 M IH0 K K L AA1 K/
noun
  1. 1

    An extremely accurate reference clock whose operation is based on an atomic process, typically the frequency of electromagnetic radiation associated with a specified energy-level transition in an element such as cesium.

  2. 2

    A radio clock.

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Source: https://en.wiktionary.org/wiki/atomic%20clock