[ 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.


Nearby words

  1. fiske,
  2. fiske, john,
  3. fiske, minnie maddern,
  4. fissi-,
  5. fissile,
  6. fission bomb,
  7. fission fungi,
  8. fission product,
  9. fission reactor,
  10. fission track

Origin of fission

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

Can be confusedfission fusion Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for fission

British Dictionary definitions for fission


/ (ˈfɪʃən) /


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 for fission

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

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

Medicine definitions for fission


[ fĭshən ]


The act or process of splitting into parts.
The amitotic division of a cell or its nucleus.
An asexual process of reproduction in which a unicellular organism divides into two or more independently maturing daughter cells.
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.

Science definitions for fission


[ fĭshən ]

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.
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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.