The second, symbol s, is the SI base unit of time.
The second 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.
The definition of the second implies the exact relation ΔνCs = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the defining constant ΔνCs :
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.
For historical reasons, the second was originally defined as 1⁄86 400 of a day. One day being the time taken for the Earth to complete one rotation about its axis.