It is currently defined as the length of the path travelled by light in a vacuum in 1/299.792.458 of a second.
The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the meter is a measure of proper length.
The speed of light in vacuum, commonly denoted c, is a universal physical constant.
A physical constant is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is contrasted with a mathematical constant, which has a fixed numerical value, but does not directly involve any physical measurement.
There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constantG, the Planck constanth, the electric constantε0, and the elementary chargee
According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any information can travel through coordinate space.
Earth is 24,901 miles around at its center. If our world had no atmosphere (air refracts and slows down light a little bit), a photon skimming along its surface could lap the equator nearly 7.5 times every second.
All moonlight that we see is 1.255 seconds old.