fission

American  

[fish-uhn] / ˈfɪʃ ən /

noun

1. the act of cleaving or splitting into parts.

2. Also called nuclear fission.  Physics. the splitting of the nucleus of an atom into nuclei of lighter atoms, accompanied by the release of energy.

3. Biology. the division of an organism into new organisms as a process of reproduction.

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verb (used without object)

1. Physics. to undergo fission.

verb (used with object)

1. Physics. to cause to undergo fission.

fission British  

/ ˈfɪʃən /

noun

1. the act or process of splitting or breaking into parts

2. biology a form of asexual reproduction in single-celled animals and plants involving a division into two or more equal parts that develop into new cells

3. short for nuclear fission

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fission Scientific  

/ fĭsh′ən /

1. The splitting of an unstable atomic nucleus into two or more nuclei. Fission occurs spontaneously, generally when a nucleus has an excess of neutrons, resulting in the inability of the strong force to bind the protons and neutrons together. The fission reaction used in many nuclear reactors and bombs involves the absorption of neutrons by uranium-235 nuclei, which immediately undergo fission, releasing energy and fast neutrons.

2. Compare fusion

3. A process of asexual reproduction in which a single cell splits to form two identical, independent cells. In fission, the chromosomal DNA replicates before the cell divides. Most bacteria and other prokaryotes reproduce by means of fission.

4. Also called binary fission

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Etymology

Origin of fission

First recorded in 1835–45; from Latin fissiōn- (stem of fissiō ) “a splitting, dividing,” equivalent to fiss(us) “divided” + -iōn- noun-forming suffix; see fissi-, -ion; fission def. 2 was first recorded in 1935–40

Explanation

Any type of dividing or splitting can be called fission. We often equate it with the splitting of atoms, which is called nuclear fission. The word fission has always been used for some type of division, beginning with the Latin fissionem, from the root findere, “to split.” As the word worked its way through history, it was applied to various areas. In 1841, the discovery of cell division was labeled fission, while the application in nuclear physics came about in 1939 with the development of the first nuclear bomb. Ironic — the word can be applied to both creating life and ending it.

Example Sentences

“There’s really strong bipartisan support for regulation on fusion power plants that does not mirror what’s done on fission power plants,” Beyer said.

From MarketWatch • May 18, 2026

Existing U.S. nuclear plants, which provide an estimated 18% of the country’s electricity, rely on fission, which splits atoms.

From MarketWatch • May 18, 2026

In one scenario, a rapidly spinning massive star explodes and splits into two smaller neutron stars through a process called fission.

From Science Daily • Apr. 24, 2026

Unlike fission, nuclear fusion generates “no long-lived radioactive waste,” Bechtel says, and unlike fossil fuels, it doesn’t involve burning finite resources and creating carbon emissions.

From The Wall Street Journal • Apr. 6, 2026

Traveling a bit more slowly, they’d be more likely to hit the uranium atoms and cause fission.

From "Bomb" by Steve Sheinkin