Adopted in France in the late 18th century, the meter is used today by much of the technologically developed world, making it an extremely successful example of both standardization and cooperation. However, the definition of the meter has changed several times. It was originally based, somewhat impractically, on a fraction of the Earth's circumference and was later defined as the distance between two scratches on a bar of platinum-iridium alloy. What is the most recent definition of the meter? More...

Definition agreed by the 26th CGPM (November 2018), implemented 20 May 2019:

The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency DeltanuCs.

This definition implies the exact relation c = 299 792 458 m s–1. Inverting this relation gives an exact expression for the metre in terms of the defining constants c and DeltanuCs:

The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of 1/299 792 458 of a second.

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