Caesium standard - Definition and Overview

In telecommunication, a caesium standard is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency.

Note: The energy level between the two hyperfine ground states corresponds, in the absence of external influences (e.g., the Earth's magnetic field), to a frequency of 9,192,631,770 Hz.

Source: from Federal Standard 1037C and from MIL-STD-188