The femtometre (American spelling femtometer) symbol fm derived from the Danish and Norwegian word femten 'fifteen', Ancient Greek: μέτρον, romanized: metrοn, lit. 'unit of measurement') is an SI unit of length equal to 10−15 metres, which means a quadrillionth of one metre. This distance used to be called a fermi and was so named in honour of Italian-American physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.
