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The millimetre is a unit of length in the metric system, equivalent to one thousandth of a metre (the SI base unit of length).

One meter was defined in 1983 by the 17th conference of weights and measures as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second” and the millimetre by definition is derived as being 1/1000th of that value.