adjective Chemistry.
noting or pertaining to a chemical change that is accompanied by a liberation of heat (opposed to endothermic ).
Also, exothermal.

1880–85; exo- + thermic

exothermically, exothermally, adverb
exothermicity [ek-soh-ther-mis-i-tee] , noun
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World English Dictionary
exothermic or exothermal (ˌɛksəʊˈθɜːmɪk)
endothermic Compare exoergic (of a chemical reaction or compound) occurring or formed with the evolution of heat
exothermal or exothermal
exo'thermically or exothermal
exo'thermally or exothermal

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Word Origin & History

from Fr. exothermique (1879), from Gk. exo- + therme "heat" (see thermal).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

exothermic ex·o·ther·mic (ěk'sō-thûr'mĭk) or ex·o·ther·mal (-məl)

  1. Of or relating to a chemical reaction during which heat is released.

  2. Of of relating to the external warmth of the body.

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
exothermic   (ěk'sō-thûr'mĭk)  Pronunciation Key 
Relating to a chemical reaction that releases heat. Compare endothermic.
The American Heritage® Science Dictionary
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Example sentences
When water is formed by burning hydrogen, that's an exothermic reaction.
Highly exothermic chemical reactions are needed to thrust spacecraft into the
Since this reaction is exothermic--that is, it releases heat--it requires no
  input of energy to drive it.
What is the difference between an endo and exothermic reaction.
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