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.

1835–45; < Latin fissiōn- (stem of fissiō) a splitting, dividing, equivalent to fiss(us) divided (see fissi-) + -iōn- -ion

fission, fusion. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
fission (ˈfɪʃən)
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
[C19: from Latin fissiō a cleaving]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1841, "division of a cell or organism," from L. fissionem (nom. fissio) "a breaking up, cleaving," from root of findere "to split" (see fissure). Cognate with O.E. bitan "to bite." Nuclear physics sense is 1939.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

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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American Heritage
Science Dictionary
fission   (fĭsh'ən)  Pronunciation Key 

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  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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Example sentences
In the photo above, one desmid is splitting into two by a process called
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.
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