Femto-
Femto- is a unit prefix in the metric system denoting a factor of 10−15. It was added in 1964 to the SI. It is derived from the Danish word femten, meaning "fifteen".
Examples of use:
The femtometre shares the unit symbol 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.
