Femto-

Femto- (symbol f) 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'. Femto- (symbol f) 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'.