World English Dictionary
nuclear reaction
a process in which the structure and energy content of an atomic nucleus are changed by interaction with another nucleus or particle

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
nuclear reaction  
A process, such as fission, fusion, or radioactive decay, in which the structure of an atomic nucleus is altered through release of energy or mass or by being broken apart. See more at fission, fusion.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

nuclear reaction definition

A reaction that changes the number of protons or neutrons in the nucleus of an atom. There are several kinds of nuclear reactions, including the fragmentation of large nuclei into smaller ones (nuclear fission), the building up of small nuclei into larger ones (nuclear fusion), and changes begun by collisions with elementary particles or other nuclei (as in particle accelerators).

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Encyclopedia Britannica

nuclear reaction

change in the identity or characteristics of an atomic nucleus, induced by bombarding it with an energetic particle. The bombarding particle may be an alpha particle, a gamma-ray photon, a neutron, a proton, or a heavy ion. In any case, the bombarding particle must have enough energy to approach the positively charged nucleus to within range of the strong nuclear force.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
The star continues to cook lighter nuclei into progressively heavier elements,
  but each nuclear reaction runs its course faster.
With all the fuel piled up at the bottom, there is some danger that the nuclear
  reaction could have restarted.
Boron absorbs neutrons and helps slow down the nuclear reaction, thus
  indirectly reducing the heat being generated.
Ratios of other xenon isotopes also point to a fast nuclear reaction that
  involved uranium.
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