The femtometre (American spelling femtometer), symbol fm,[1][2] (derived from the Danish and Norwegian word femten 'fifteen', Ancient Greek: μέτρον, romanized: metron, lit.

This distance is sometimes called a fermi and was so named in honour of Italian naturalized to American physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.

[4] 1 barn = 100 fm2 The femtometre was adopted by the 11th Conférence Générale des Poids et Mesures, and added to the SI in 1964, using the Danish word for "15" and the similarity in spelling with fermi.

The term was coined by Robert Hofstadter in a 1956 paper published in Reviews of Modern Physics entitled "Electron Scattering and Nuclear Structure".

When Hofstadter was awarded the 1961 Nobel Prize in Physics, it subsequently appeared in the text of his 1961 Nobel Lecture, "The electron-scattering method and its application to the structure of nuclei and nucleons" (December 11, 1961).