What is "um" in measurements?

The term "um" is used as an abbreviation for the micrometer, although the proper abbreviation is the Greek symbol mu in place of the u. A micrometer is also referred to as a micron.

A meter contains 1 million micrometers. Similarly, a millimeter contains 1,000 micrometers or 39 millionths of an inch. The micrometer is used in physics for the study of wavelengths, in astronomy for the measure of angles and degrees, and in cellular biology. It is sometimes used in manufacturing, such as in grading wool fibers. Units smaller than a micrometer include nanometers, angstroms, picometers, zeptometers and yoctometers.