What is the X in X-mas?
(US "meter") The fundamental SI unit of length.
From 1889 to 1960, the metre was defined to be the distance between two scratches in a platinum-iridium bar kept in the vault beside the Standard Kilogram at the International Bureau of Weights and Measures near Paris.
This replaced an earlier definition as 10^-7 times the distance between the North Pole and the Equator along a meridian through Paris; unfortunately, this had been based on an inexact value of the circumference of the Earth.
From 1960 to 1984 it was defined to be 1650763.73 wavelengths of the orange-red line of krypton-86 propagating in a vacuum.
It is now defined as the length of the path traveled by light in a vacuum in the time interval of 1/299,792,458 of a second.