What does femtosecond mean?
A femtosecond is an SI unit of time equal to one quadrillionth of a second — that is, 10⁻¹⁵ seconds. It belongs to the family of metric prefixes built on 'second', sitting between the picosecond above it and the attosecond below it. To grasp its scale: light travels only about 300 nanometres, roughly the size of a small virus, in a single femtosecond. The unit is indispensable in modern science, particularly in ultrafast laser physics, femtosecond spectroscopy, and femtochemistry, where researchers watch chemical bonds break and form in real time; Ahmed Zewail's pioneering work in this field earned the 1999 Nobel Prize in Chemistry. Femtosecond lasers are also used clinically, notably in LASIK eye surgery. Outside technical contexts the word occasionally serves as a vivid hyperbole for something happening almost instantaneously. Its plural is femtoseconds.
nounOne quadrillionth of a second (10⁻¹⁵ seconds); an SI unit of time used chiefly in physics and chemistry to measure extremely brief events such as molecular vibrations and laser pulses.
- One quadrillionth of a second, an SI unit of time equal to 10⁻¹⁵ seconds.
"The chemical bond broke within a few femtoseconds of absorbing the photon."
"Electron movements inside atoms occur on the attosecond scale, while chemical reactions typically unfold over tens to hundreds of femtoseconds."
"Femtosecond lasers have revolutionised eye surgery by removing tissue with almost no heat damage."
Regular plural; commonly preceded by numerals in scientific writing, e.g. '50 fs'.
"The reaction completed in under 200 femtoseconds."
A femtosecond is to a second what a second is to about 32 million years — which is why Ahmed Zewail's femtochemistry won the Nobel Prize.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of femtosecond
The word combines 'femto-', the SI metric prefix denoting 10⁻¹⁵, with 'second', the base unit of time. The prefix 'femto' was adopted by the General Conference on Weights and Measures in 1960 and derives from Danish or Norwegian 'femten', meaning fifteen, reflecting the power of ten involved. It follows the same Scandinavian-derived pattern as its companion prefix 'atto', from 'atten' (eighteen).
How femtosecond is actually used
A technical term confined mainly to physics, chemistry, optics, and engineering; in everyday speech it appears only figuratively or humorously to suggest extreme brevity.
Easily confused with femtosecond
A picosecond is a trillionth of a second (10⁻¹² s), one thousand times longer than a femtosecond (10⁻¹⁵ s).