Noun Concept
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atomic clock  Atomic second  Atom clock  Atom clocks  Atomic clocks
A timepiece that derives its time scale from the vibration of atoms or molecules WordNet 3.0
An atomic clock is a clock device that uses a hyperfine transition frequency in the microwave, or electron transition frequency in the optical or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Wikipedia
Extremely accurate reference clock used as a standard for timekeeping Wikidata
A clock that uses an electronic transition frequency for its timekeeping element. OmegaWiki
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. Wiktionary
Reference clock. Wiktionary (translation)