A metre (or meter) is the base unit of length in the International System of Units (SI). It is defined as the distance that light travels in a vacuum in 1/299,792,458 seconds. This definition has been in place since 1983, and it reflects a fundamental constant of nature, making it a highly precise standard for measurement.
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The metre, symbol , is the SI base unit of measurement for length. It is defined by fixing the speed of light in vacuum to , with the second defined independently. Thus light in vacuum travels one metre in of a second.
The exact defining constant does not make every measured length exact. Realizing a length standard can involve optical interferometry, frequency references, alignment and corrections for propagation through air; their imperfections contribute to measurement uncertainty. The definition specifies the reference, whereas an experiment supplies an imperfect realization.[ref]
The International System of Units (SI), ninth edition, SI Brochure. section 2.3.1: definition of the metre