Source: NIST

The second is the unit of time in the International System of Units (SI). It is one of the seven SI base units. The unit of time, the second, was defined originally as the fraction 1/86,400 of the mean solar day. The exact definition of "mean solar day" was left to astronomical theories. However, measurement showed that irregularities in the rotation of the Earth could not be taken into account by the theory and have the effect that this definition does not allow the required accuracy to be achieved. In order to define the unit of time more precisely, the 11th General Conference on Weights and Measures (CGPM) (1960) adopted a definition given by the International Astronomical Union which was based on the tropical year. Experimental work had, however, already shown that an atomic standard of time-interval, based on a transition between two energy levels of an atom or a molecule, could be realized and reproduced much more precisely. Considering that a very precise definition of the unit of time is indispensable for the International System, the 13th CGPM (1967) decided to replace the definition of the second by the following (affirmed by the International Committee for Weights and Measures (CIPM) in 1997 that this definition refers to a cesium atom in its ground state at a temperature of 0 K):

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Further reading
The NIST Reference on Constants, Units, and Uncertainty

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