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unit (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.