1 metre can be defined as:
Distance travelled by light in (1/299792458) s
The speed of light is constant throughout the vacuum. Hence it is a suitable standard and can be taken as reference for measurement of distance. The speed of light is 299792458 ms−1 in a vacuum.
We know, speed=distancetime⇒time=distancespeed
Given, distance =1 m
∴time=1299792458 s
i.e., One meter is the distance travelled by light in 1299792458 s in vacuum.