femto

Femto- is a prefix in the International System of Units (SI) denoting a factor of 10^-15. Its symbol is f. The prefix is used to form names of SI units to express values that are one quadrillionth of the base unit. For example, a femtometre (fm) equals 10^-15 metres, and a femtosecond (fs) equals 10^-15 seconds. The prefix can be applied to any SI unit, yielding units such as femtograms (fg) and femtoliters (fL).

Common contexts include physics, chemistry, and engineering, particularly where very small lengths or very short times are involved. In ultrafast optics, femtosecond laser pulses enable femtosecond spectroscopy; in nuclear and particle physics, femtometer-scale lengths describe nuclear dimensions.

History and standardization: Femto- is one of the SI prefixes standardized by the General Conference on Weights and Measures (CGPM) and implemented by the International Bureau of Weights and Measures (BIPM). It was introduced in the 20th century as part of the system of prefixes for decimal multiples and submultiples of SI units.

See also: pico-, nano-, micro-, milli-.