1) Metric linear unit 2) Millimetre 3) Small unit 4) Very small measure of length 1) Millimetre 2) Mm 
Millimeter (mm), Metric unit of length, equal to 0.04 inch (there are 25 mm in an inch). There are 1000 millimeters in a meter.
• (n.) Alt. of Millimetre
A measure of length in the metric system. A millimeter is one thousandth of a meter. There are 25 millimeters in an inch.
One-thousandth of a meter. ... Abbreviation: mm ... (05 Mar 2000) ... 
(mm) (mil´ĭ-me″tәr) one thousandth (10−3) of a meter; equivalent to 0.039 inch.
10-3 meter. 
(mm) Type: Term Pronunciation: mil′i-mē-tĕr Definitions: 1. One-thousandth of a meter.
[n] - a metric unit of length equal to one thousandth of a meter
One thousandth of a meter, symbol mm. 
millimetre noun a metric unit of length equal to one thousandth of a meter
(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.