from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun The act or process of splitting into parts.
  • noun A nuclear reaction in which an atomic nucleus, especially a heavy nucleus, such as an isotope of uranium, splits into fragments, usually two fragments of comparable but unequal mass, and releases a few neutrons and about 100 million electron volts of energy. Nuclear fission may occur spontaneously or may be induced by the absorption of a neutron, which can initiate a nuclear chain reaction.
  • noun Biology An asexual reproductive process in which a unicellular organism divides into two or more independently maturing daughter cells.
  • intransitive verb To cause (an atom) to undergo fission.
  • intransitive verb To undergo fission.

from The Century Dictionary.

  • noun The act of cleaving, splitting, or breaking up into parts.
  • noun In biology, the automatic division of a cell or an independent organism into new cells or organisms; especially, such division as a process of multiplication or reproduction. Also fissuration. See cut under Paramecium.

from the GNU version of the Collaborative International Dictionary of English.

  • noun A cleaving, splitting, or breaking up into parts.
  • noun (Biol.) A method of asexual reproduction among the lowest (unicellular) organisms by means of a process of self-division, consisting of gradual division or cleavage of the into two parts, each of which then becomes a separate and independent organisms; as when a cell in an animal or plant, or its germ, undergoes a spontaneous division, and the parts again subdivide. See Segmentation, and Cell division, under Division.
  • noun (Zoöl.) A process by which certain coral polyps, echinoderms, annelids, etc., spontaneously subdivide, each individual thus forming two or more new ones. See Strobilation.
  • noun (Physics) The act or process of disintegration of an atomic nucleus into two or more smaller pieces; called also nuclear fission. The process may be spontaneous or induced by capture of neutrons or other smaller nuclei, and usually proceeds with evolution of energy.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun The process whereby one item splits to become two.
  • noun physics The process of splitting the nucleus of an atom into smaller particles; nuclear fission
  • noun biology The process by which a bacterium splits to form two daughter cells.
  • verb To cause to undergo fission.
  • verb intransitive To undergo fission.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun a nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy
  • noun reproduction of some unicellular organisms by division of the cell into two more or less equal parts


from The American Heritage® Dictionary of the English Language, 4th Edition

[Latin fissiō, fissiōn-, a cleaving, from fissus, split; see fissi–.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

From Latin fissiōnem, accusative singular of fissiō ("the act of breaking up"), from findō ("split, divide").


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  • "He (American biologist William A. Arnold) could have gone to Berkeley to pick up radioisotope technique, but would have missed living in Copenhagen, learning from de Hevesy - would have missed contributing a coinage to the gamble that is history.... 'Later that day Frisch looked me up and said, 'You work in a microbiology lab. What do you call the process in which one bacterium divides into two?' And I answered, 'binary fission'. He wanted to know if you could call if 'fission' alone, and I said you could'"

    -- From "The Making of the Atomic Bomb" by Richard Rhodes, Frisch referring to Otto Frisch, who with Lise Meitner first postulated nuclear fission.

    November 9, 2007