The unperturbed ground state hyperfine transition frequency of the caesium 133 atom

SI defining constant
Name	Symbol	Value	Unit		Expressed in terms of SI base units
the unperturbed ground state hyperfine transition frequency of the caesium 133 atom	ΔνCs	9 192 631 770	Hz		s−1
Definition The numerical value of the frequency of the radiation that is produced when an unperturbed atom of caesium-133 undergoes a hyperfine transition in its ground state is defined to be exactly 9 192 631 770 when expressed in the unit hertz, or s-1. It is denoted by the symbol ΔνCs.

The hyperfine transition frequency of the caesium 133 atom, Δν_Cs, forms the basis for the definition of the second.

Nature

Caesium-133 is the only stable isotope of caesium (element atomic number 55).

The unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, symbol Δν_Cs, has the character of an atomic parameter, which may be affected by the environment, such as electromagnetic fields. However, the underlying transition is well understood, stable and a good choice as a reference transition under practical considerations.

The choice of an atomic parameter like Δν_Cs does not disconnect definition and realisation in the same way that h, c, e, or k do, but specifies the reference.