Answer:
One second is defined by taking the fixed numerical value of the caesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 (9.192631770 x 10⁹ ) cycles of the radiation, when expressed in the unit Hz, which is equal to s−¹.
One second is equal to 1/86,400 of a mean solar day.