Micrometre
The micrometre (International spelling as used by the International Bureau of Weights and Measures;SI symbol: µm) or micrometer (American spelling), also commonly known as a micron, is an SI derived unit of length equaling 1×10−6 of a metre (SI standard prefix "micro-" = 10−6); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch). The symbol µm is sometimes rendered as um if the symbol µ cannot be used, or if the writer is not aware of the distinction.
The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria and is also commonly used in plastics manufacturing. Micrometres are the standard for grading wool (referring to the diameter of wool fibres). Any wool finer than 25 µm can be used for garments, whilst coarser grades are used for outerwear, rugs, and carpets. A human hair ranges from 17 to 181 µm.
The symbol for the SI prefix micro- is a Greek lowercase mu. In Unicode, it has the codepoint U+00B5 (µ), distinct from the codepoint U+03BC (μ) of the Greek letter lowercase mu, so that machines can recognize it as the SI prefix symbol rather than as a letter. Most fonts use the same glyph for the two characters.