SI base unit: second (s)

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δν_Cs, 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.

This definition implies the exact relation Δν_Cs = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the defining constant Δν_Cs:

or

The effect of this definition is that the second is equal to the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the ¹³³Cs atom.

See: