The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels (F = 4, mF = 0 to F = 3, mF = 0) of the ground state of the caesium 133 atom.
Citing this page:
Generalic, Eni. "Second." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.