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Metre
In SI units, the metre (symbol m) is the distance travelled by light in vacuum in 
second.
Explanation
A second is defined by a hyperfine transition frequency of caesium.
HistoryThe metre was originally defined in 1791 as one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris, setting 10 000 km as that quarter of the Earth's polar circumference. |