What does fission mean?
Fission is the act or process of splitting something into parts, a term now dominated by its use in nuclear physics. There, fission describes what happens when a heavy atomic nucleus — typically of uranium or plutonium — absorbs a neutron and breaks apart into smaller nuclei, releasing a large amount of energy along with further neutrons that can sustain a chain reaction. This reaction powers nuclear reactors and, in uncontrolled form, atomic weapons. In biology, fission has an older and quieter meaning: the division of a single-celled organism into separate cells, most familiarly in the phrase 'binary fission', the reproductive mode of bacteria and amoebae. The word carries a formal, technical register and rarely appears in casual conversation outside its nuclear sense. Its Latin root, meaning to split or cleave, links it to words such as fissure and fissile. Fission remains central to debates on energy, weapons, and science policy, making it a word of both precise technical meaning and broad cultural weight.
nounThe process of splitting into two or more parts, especially the splitting of a heavy atomic nucleus (such as uranium-235) into smaller nuclei with the release of enormous energy. In biology, it also refers to the division of a single-celled organism into two or more daughter cells.
- Physics: the splitting of a heavy atomic nucleus into two or more lighter nuclei, releasing energy and neutrons.
- Biology: the reproduction or division of a single-celled organism into two or more parts, each growing into a new organism (e.g. binary fission).
"Nuclear fission releases vast amounts of energy from a single atom of uranium."
"The reactor harnesses controlled nuclear fission to generate electricity for millions of homes."
"Amoebae multiply through binary fission, dividing into two identical daughter cells."
Rarely pluralized; the singular form is normally treated as a mass process. When counted at all, 'fissions' appears mainly in technical physics writing referring to individual splitting events.
"Each pulse of the reactor produced billions of individual fissions."
The word 'fission' was borrowed from Latin in the 1600s for ordinary splitting — and then became one of the most consequential words of the 20th century when physicists split the atom in 1938.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of fission
Fission derives from the Latin 'fissionem', meaning a cleaving or breaking in two, formed from the verb 'findere' ('to split'). English adopted the word in the early seventeenth century as a general term for the act of splitting. It shares this root with cognates such as fissure, fissile, and fissiparous. The word acquired its famous modern sense after Otto Hahn and Fritz Strassmann's discovery of nuclear splitting in 1938, which Lise Meitner and Otto Frisch interpreted — with Frisch coining 'nuclear fission' by analogy with biological cell division.
Related word forms
How fission is actually used
Predominantly a scientific term used in formal and technical contexts; in everyday language it appears almost exclusively in compounds like 'nuclear fission'. In physics writing it is often paired with 'fusion' as its counterpart, and the contrast between the two is a frequent source of public confusion.
Easily confused with fission
Fission splits an atomic nucleus apart, while fusion joins two light nuclei together — both release energy, but by opposite processes.
A fissure is a physical crack or split in rock or tissue, whereas fission is the process or act of splitting, especially at the atomic or cellular level.