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fis·sion

(fĭshən, fĭzh-)

1. The act or process of splitting into parts.

2. 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 along with a few neutrons, releasing energy in the order of magnitude of 100 million electron volts. Nuclear fission may occur spontaneously or may be induced by the absorption of a neutron, which can initiate a nuclear chain reaction.

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

v. fis·sioned, fis·sion·ing, fis·sions

v. tr.

To cause (an atom) to undergo fission.

v. intr.

To undergo fission.

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

Indo-European & Semitic Roots Appendices

Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

Indo-European Roots

Semitic Roots

The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.

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