numerical value of the speed of light in vacuum, ${\displaystyle \mathrm {c} }$, to be ${\displaystyle 299\ 792\ 458}$ when expressed in the unit ${\displaystyle \mathrm {m\ s^{-1)) }$, where the second is defined in terms of the caesium frequency ${\displaystyle \mathrm {\Delta \nu _{Cs)) }$."