The speed of light in vacuum is 299,792,458 m/s. Then time taken by light to cover a distance of 1 meter in vacuum =1/(299 ,792 ,458) s. Hence, the meter is defined as the distance travelled by light in 1/(299, 792, 458) s. As 300,000,000 m/s is an approximate speed of light in vacuum, it cannot be used to define the meter. The distance travelled by light in one second is 299,792,458 m. This is a large quantity and cannot be used as a base unit. So, the meter is not defined in terms of second.