A measure of length in the metric system. A millimeter is one thousandth of a meter. There are 25 millimeters in an inch. Found on http://www.cancer.gov/dictionary?expand=M
(mm) One millimeter is equal to one one-thousandth (1/1,000) of a meter. A dime is slightly thicker than one mm. About 25 mm equal one inch. Found on https://www.health.ny.gov/environmental/glossary/