Mm is a unit of length, which refers to millimeters. The unit of length refers to the basic unit for measuring spatial distance, which is formulated by human beings to standardize the length. Its international unit is meter (symbol M), and commonly used units are millimeter (mm), centimeter (cm), decimeter (dm), kilometer (km), meter (m), micron (micron), nanometer (nm) and so on. Length units play an important role in various fields.

In measurement, a certain kind of quantity is taken as a reference quantity to determine how many times the known quantity is equivalent to the reference quantity, which is called a unit. For example, centimeter is the unit for measuring length, gram is the unit for measuring mass, and second is the unit for measuring time. As an example of mathematics, it can also be said that 1 is the unit of natural number, and the fraction with the numerator of 1 is the unit of fraction.

When measuring the length of a line segment, you need to select a line segment as the measurement standard and specify its length as 1, which is called the length unit. There are many units in length, the main unit is meter, and the rest are derived units. When measuring, choose the appropriate length unit according to the actual needs. For example, the length of the runway is in meters, the distance between two cities is in kilometers, and the distance of long jump is in centimeters.