The length of an object is a one-dimensional linear measurement of its longest side. In the physical sciences, length and distance are synonymous and the SI (abbreviation from the French Le Système International d’Unités) unit of length is the metre.

In 1983 the definition of the metre was fixed in terms of time and the speed of light whereby the metre is the length of the path travelled by light in vacuum during a time interval of 1⁄299 792 458 of a second.