What does fermi mean?
The fermi is a unit of length equal to one femtometre, that is, 10⁻¹⁵ of a metre, and it belongs to the vocabulary of nuclear physics rather than everyday life. At this scale, physicists describe the dimensions of atomic nuclei: a typical nucleus measures a few fermis across, and even a proton spans less than two. The unit was named in honour of Enrico Fermi, the Italian-American physicist who produced the first controlled nuclear chain reaction, and it came into use among American scientists in the mid-twentieth century. Although the International System of Units formally designates this length as the femtometre, the older name survives in textbooks, research papers, and the working speech of nuclear and particle physicists. For anyone reading about nuclear structure, encountering distances quoted in fermis is almost unavoidable, making it a small but genuinely useful term to know.
nounA unit of length equal to one femtometre (10⁻¹⁵ metre), used chiefly in nuclear physics to express the sizes of atomic nuclei. It is named after the Italian physicist Enrico Fermi.
- A unit of length equal to 10⁻¹⁵ metre (one femtometre), used for measuring nuclear dimensions.
"The radius of a proton is roughly 0.8 fermis."
"Nuclear radii are typically a few fermis across."
"The experiment probed distances down to fractions of a fermi."
Regular English plural, used when quoting quantities of more than one unit.
"The nucleus measured about six fermis in diameter."
An entire atomic nucleus fits inside a span of just a few fermis — a unit so small that a metre holds a quintillion of them, and it's named for the physicist who built the world's first nuclear reactor.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of fermi
The word is an eponym honouring Enrico Fermi (1901–1954), the Italian-born physicist who led the construction of the first nuclear reactor. The unit was adopted by nuclear physicists in the United States in the years following his work, as a convenient name for the scale of nuclear dimensions. The same scientist also gave his name to the element fermium and to the particles known as fermions.
How fermi is actually used
The term is somewhat dated in formal metrology: 'femtometre' is the preferred SI name, but 'fermi' remains common in older nuclear-physics literature and colloquially among physicists. The plural is 'fermis'.
Easily confused with fermi
Fermium is a synthetic chemical element (number 100) named for the same physicist, whereas the fermi is a unit of length equal to a femtometre.