- a unit of length equal to one thousandth of a meter and equivalent to 0.03937 inch. Abbreviation: mm
n.also millimetre, 1802, from French millimetre; see milli- + meter (n.2). n.
- A unit of length equal to one thousandth (10-3) of a meter (0.0394 inch).
- A unit of length in the metric system equal to 0.001 meter. See Table at measurement.