A cesium standard is a highly constant frequency standard whose frequency is derived from the electronic transition between two hyperfine levels of the cesium-133 atom.
The change of state between the two levels occurs at 9,192,631,770 hertz( Hz), 9.192... GHz. This frequency could be measured in the 1960s and the second defined in the System of Units( SI) was derived from it.
Cesium standards belong to the measurement standards and have an extremely high frequency stability of `10^-11` to `10^-12`. The frequency drift is `10^-13`. The high frequency constancy is guaranteed when no external influences by the earth's magnetic field or electromagnetic interference fields affect the cesium standard.