Definition Of The Metre
This was adopted in competition with the definition of
The definitions are remarkably similar because of the coincidence that $\pi^2$ is very nearly equal to g, acceleration due to gravity.
However, both definitions have problems. You can read more about them in the Wikipedia article quoted below.
More recently, the metre has been redefined as:
Thus the speed of light is now, by definition, 299 792 458 m/s.