A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length. It is defined as one millionth of a metre (1×10−6 m), equivalent to one thousandth of a millimetre.
The symbol µ (Unicode character U+00B5; HTML µ) is the "micro sign"; it is derived from 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, e.g. 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 business) use the old name micron and/or the solitary symbol µ for the same thing, even though it is officially discouraged. They were official from 1879-1967.
A Micrometer is also a name for a measuring instrument.
nanometre << micrometre << millimetre
See also