A unit of length in the metric system equal to one-thousandth of a meter, abbreviated as 'mm.'
From the metric prefix 'milli-' (Latin for 'thousand') combined with 'meter' (from Greek 'metron,' meaning measure). The metric system was standardized in France during the 1790s.
The millimeter was supposed to be one ten-millionth of the distance from the Earth's equator to the North Pole, making the whole metric system literally based on planet Earth—pretty ambitious for a definition!
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