A micrometre (or micrometer) is 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 inches). Its unit symbol in the International System of Units (SI) is µm. The latter may be rendered as um if Greek fonts are not available or not admissible.
The micrometre is a common unit of measurement for wavelengths of infrared radiation and well as sizes of cells and bacteria.
The symbol for the SI prefix micro-, µ, came from the Greek lowercase mu, μ. Today it has its own Unicode point, distinct from Greek lowercase mu, to acknowledge and encode its semantic power. The two characters are usually displayed with the same glyph in most fonts, so many users are not aware of the encoding difference.
Read more about Micrometre: ISI Standardization