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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.

The metre is a unit of length in the metric system, and is the base unit of length in the International System of Units (SI).

As the base unit of length in the SI and other m.k.s. systems (based around metres, kilograms and seconds) the metre is used to help derive other units of measurement such as the newton, for force.