A millimeter (mm) is a unit of length in the metric system of measurement. It is of a meter, the base unit of length in the International System of units (SI).
Like other units in SI, the millimeter follows a system prefixes; in this case, "milli-" which stems from the Latin "mille" meaning one thousand. It follows that:
1,000 millimeters = 1 meter
1 millimeter = 0.001 meters
Refer to the metric system for more on SI prefixes. SI prefixes can also be referred to as metric prefixes, since SI is just the modern form of the metric system and includes many metric units.
Rulers that have centimeters also usually have millimeter markers between the centimeters. 1 cm = 10 mm. The figure below shows rulers that have millimeter, centimeter, and inch markings. On the rulers, the smallest markings (in terms of length and based on the width between each marking) indicate millimeters. Every fifth millimeter has a slightly longer marking, and each centimeter is indicated by a marking that is also slightly longer.
Did you know?
The smallest widths the human eye can see are around 0.02 to 0.04 millimeters, which is around the width of a human hair. There are many things in the world that are much smaller that the human eye cannot see!