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Length is a one-dimensional measure (in contrast, surface is a two-dimensional measurement while volume is three-dimensional.)

Among the base units of length is the metre, the definition of which has changed through history. At first, the metre was defined to be one ten-millionth of the distance between the Earth's equator and the North Pole. Currently, the metre is defined as “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.”