Look up femto- in Wiktionary, the free dictionary.
Femto- (symbol f) is a prefix in the metric system denoting a factor of 10−15 or 0.000000000000001. Adopted by the 11th Conférence Générale des Poids et Mesures, it was added in 1964 to the SI. It is derived from the Danish word femten, meaning "fifteen".
Example of use:
a proton has a diameter of about 1.6 to 1.7 femtometres.
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.