https://doi.org/10.1351/goldbook.S05513

The second, symbol \(\text{s}\), is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency \(\Delta\nu_\text{Cs}\), the 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 \(\text{s}^{-1}\).*Source: *

BIPM, The International System of Units, SI Brochure, 9^{th} ed. (2019), p. 130 (https://www.bipm.org/en/publications/si-brochure/)

BIPM, The International System of Units, SI Brochure, 9