second

https://doi.org/10.1351/goldbook.S05513
SI base unit of time (symbol: \(\text{s}\)). The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the @G02704@ of the caesium-133 atom.
Sources:
Green Book, 2nd ed., p. 70 [Terms] [Book]
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 991 [Terms] [Paper]