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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. 2018.
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Examples from the Web for fission
Contemporary Examples
Historical Examples
  • The multiplication of the amba is brought about by a process of fission or division.

    The Sea Shore William S. Furneaux
  • The foraminifer multiplies by fission, or by a process of budding.

    The Sea Shore William S. Furneaux
  • They are contractile, and propagated by spontaneous division, or fission.

    The Ocean World: Louis Figuier
  • It will be found full of septic vibrios in process of generation by fission.

    Louis Pasteur Ren Vallery-Radot
  • As already said, the lowest form of reproduction is that by fission.

    Evolution Joseph Le Conte
  • At the instant of fission, some 75 radionuclides, or fission products, are created.

    Atoms, Nature, and Man Neal O. Hines
  • The amœba, like other one-celled organisms, reproduces by the process of fission.

    A Civic Biology George William Hunter
  • Just as the life of a man is only the fission of two amœbas on a larger scale.

  • Any atom might be hit, blazing, into fission and decaying into other elements.

    The Impossible Voyage Home Floyd L. Wallace
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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