UK Keratoconus Self-Help and Support Association

A micrometre (American spelling: micrometer; symbol µm) is an SI unit of length equal to one millionth of a metre, or equivalently, one thousandth of a millimetre. It is also commonly known as a micron. It can be written in scientific notation as 1×10−6 m, meaning 1 / 1,000,000 m.

The symbol µm's character µ (Unicode character U+00B5; HTML µ) is the "micro sign", which should look identical to the Greek letter mu (μ) (the two may or may not look the same, depending on the font). The symbol "um" is sometimes used, when the µ and μ are not available, for example in ASCII, or when using a typewriter.

The micrometre is a common unit of measurement for wavelengths of infrared radiation. Some people (especially in astronomy and the semiconductor industry) use the old name micron and/or the solitary symbol µ (both of which were official[citation needed] between 1879 and 1967) to denote a micrometre. This practice persists in the face of official discouragement, perhaps to help disambiguate between the unit of measurement and the micrometer, a measuring device.