- The definition of fundamental units of measure, such as the base SI units. For example, the time unit second is defined as 9,192,631,770 oscillations of cesium atoms at their natural resonance frequency.
- A reference which has been determined with complete, or nearly complete accuracy, and which serves as a primary basis for measurements. For instance, an atomic frequency standard, which can be used in atomic clocks, which in turn may be used to correct and calibrate other clocks.
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