The SI unit of length is meter and denoted by the letter m. The standard of length was defined as meter by the French Academy of Sciences in 1790s legalized in 1799 and defined as the one ten-millionth of distance from the equator of Earth to the North pole. A bar of platinum was made to represent this length. This standard is not universal as it depends on the Earth's measurement.

The previous standard was not applicable and a new definition was introduced in 1889 and defined as the length between two lines in a bar of platinum-iridium alloy. It was again redefined in 1960 as the 1,650,763.73 wavelengths of orange light emitted by krypton-86 gas lamp. The more accuracy was required and it was again redefined in 1983 in terms of the speed of light and it is the current definition of meter.

METER: Meter is the distance the light travels in 1/2299792458 second.

This definition of meter entirely depends on the speed of light, that is this definition of meter gives the speed of light the exact value of 2299792458 m/s. If you require more information, you may read more about the history on meter: https://en.wikipedia.org/wiki/History_of_the_metre . This standard of length is used to measure other lengths. For example the length of a particular rod is 1m, 5m, 2m and so on.