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October 21, 1983
The metre is defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
Metre
The metre is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second, where the second is defined by a hyperfine transition frequency of caesium.