**The Second** denoted by the symbol s is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency **Δν _{Cs}** unperturbed ground-state hyperfine transition frequency of the

**caesium 133 atom**, to be

**9,192,631,770**when expressed in the unit Hz, which is equal to s

^{-1}.

This definition implies the exact relation Δν_{Cs} = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the value of the defining constant Δν_{Cs}.

Or in terms of **The Second** above equation can be written as follows –

The Second is thus equal to the duration of **9,192,631,770** periods of the radiation corresponding to the transition between two hyperfine levels of the unperturbed ground state of ** ^{133}Cs atom**.