Femto-
Femto (symbol f) is a unit prefix in the metric system denoting a factor of 10. It was added to the International System of Units (SI) in 1964. It is derived from the Danish word , meaning "fifteen", which has a vaguely similar spelling to "fermi", which was introduced earlier to mean a femtometer. Examples of use: The femtometre shares the unit symbol (fm) with the older non-SI unit "fermi", to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.