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