A millimeter (abbreviated as mm and sometimes spelled as millimetre) is a small unit of displacement (length/distance) in the metric system. The metric system, sometimes considered synonymous with the International System of Units (SI), is used throughout the world in the sciences and is the standard of measurement for science engineering,technology and Mathematics (STEM).
The word meter is Greek for measure. Millimeters are used to measure very small but visible-scale distances and lengths. In terms of real-world comparisons, a millimeter is roughly the size of the wire used in a standard paper clip. The metric system is based on decimals: There are 10mm in a centimeter and 1000mm in a meter. The base of the Greek-rooted words indicates that they are hundredths (centi) and thousandths (milli) of meters.
Some fractions of meters: