Meter
Symbol: m
The meter (or metre) is the basic unit of length in the International System of Units.
1793 the French National Convent defined the meter to represent one ten-millionth of the distance from the Equator to the North Pole through Paris. The prototype was built in 1795 out of brass. More exact measurements of the circumference of the earth lead to a corrected version. This meter prototype was represented by two marks on a platinum-iridium bar. Until 1960 this bar was the basis for the meter definition. In 1960 the meter was defined in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.
In 1983, the meter was redefined as the distance travelled by light in free space in 1/299,792,458 of a second.