For the second (s), the unit for time, there is an astronomical and a technical definition.
For astronomical time, the second is defined as the 86,400th part of a mean solar day at a fixed location. Because the astronomical second is not constant due to variations in the Earth's rotation, the International Committee of Weights and Measures redefined the second in 1956 as 1/31,556,925.9747th of the time it took the Earth to orbit the Sun during 1900.
The atomic physics definition introduced in 1964 allowed a higher precision to be defined. The atomic physics second, also known as the SI second or atomic second, has been defined in the System of Units (SI) since 1967. The resonance frequency of a cesium atom (Cs 133) between the two hyperfine levels was defined as the reference oscillation. The second is derived from this highly constant resonance source with a resonance frequency of 9,192,631,770 GHz. Using cesium standards, long term constants of `10^14` are obtained, corresponding to a deviation of 1 second in several million years.
The 1967 definition of the second is: the second is 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine structure levels of the ground state of atoms of the nuclide 133Cs.