|Definition:|| SI unit of time, equal to 9 192 631 770 periods of the ground-state hyperfine transition of the unperturbed caesium 133 atom
Note 1 to entry: The definition in the SI Brochure is: "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 s−1."
Note 2 to entry: The reference to an unperturbed atom is intended to make it clear that the definition of the second is based on an isolated caesium atom that is unperturbed by any external field, such as ambient black-body radiation.
Note 3 to entry: As explained in IEV 113-01-13, Note 5, the quantity name “duration” is more precise than the loose term “time”.
Note 4 to entry: The second is one of the seven SI base units: second (s), metre (m), kilogram (kg), ampere (A), kelvin (K), mole (mol) and candela (cd).