The second, symbol \(\pu{s}\), is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency \(\Delta \nu_{\ce{Cs}}\), the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be \(\pu{9192631770}\) when expressed in the unit Hz, which is equal to \(\pu{s-1}\).