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[fish-uh n] /ˈfɪʃ ən/
the act of cleaving or splitting into parts.
Also called nuclear fission. Physics. the splitting of the nucleus of an atom into nuclei of lighter atoms, accompanied by the release of energy.
Compare fusion (def 4).
Biology. the division of an organism into new organisms as a process of reproduction.
verb (used without object)
Physics. to undergo fission.
verb (used with object)
Physics. to cause to undergo fission.
Origin of fission
1835-45; < Latin fissiōn- (stem of fissiō) a splitting, dividing, equivalent to fiss(us) divided (see fissi-) + -iōn- -ion
Can be confused
fission, fusion. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for fission
  • In the photo above, one desmid is splitting into two by a process called fission.
  • Some of the speeding subatomic particles of the fission process turn uranium into plutonium.
  • Relax and let the fission between you work it's magic.
  • Nuclear energy comes in two different flavors: fission and fusion.
  • The way to cut off a fission chain reaction, then, is to intercept the neutrons.
  • Upshot-Knothole involved the testing of new theories, using both fission and fusion devices.
  • The many safety mechanisms of a nuclear plant focus mainly on keeping these so-called fission products out of the environment.
  • It cannot stop a determined cheat producing a crude fission bomb, since the technology is well-known.
  • Implosion bombs create a fission reaction, which causes the nuclei of atoms to split and release huge amounts of energy.
  • When it comes to radical energy solutions, an extreme long shot is a nuclear power scheme that would combine fusion and fission.
British Dictionary definitions for fission


the act or process of splitting or breaking into parts
(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
short for nuclear fission
Word Origin
C19: from Latin fissiō a cleaving
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for fission

1841, "division of a cell or organism," from Latin fissionem (nominative fissio) "a breaking up, cleaving," from past participle stem of findere "to split" (see fissure). Cognate with Old English bitan "to bite." Nuclear physics sense is 1939. As a verb, from 1929.

Online Etymology Dictionary, © 2010 Douglas Harper
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fission in Medicine

fission fis·sion (fĭsh'ən)

  1. The act or process of splitting into parts.

  2. The amitotic division of a cell or its nucleus.

  3. An asexual process of reproduction in which a unicellular organism divides into two or more independently maturing daughter cells.

  4. A nuclear reaction in which an atomic nucleus, especially a heavy nucleus such as an isotope of uranium, splits into fragments, usually two of comparable mass, with the evolution of from 100 million to several hundred million electron volts of energy.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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fission in Science

  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. Compare fusion.

  2. 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. Also called binary fission.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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