https://doi.org/10.1351/goldbook.S05513
The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ΔνCs, 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 s1.
Source:
BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 130 [Terms] [Book]