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