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verb (used without object)
to act in response to an agent or influence: How did the audience react to the speech?
to act reciprocally upon each other, as two things.
to act in a reverse direction or manner, especially so as to return to a prior condition.
to act in opposition, as against some force.
to respond to a stimulus in a particular manner: reacting to a shock by jumping; to react to the word “coward” with anger.
to undergo a chemical reaction.

1635–45; re- + act, probably modeled on Medieval Latin reagere

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
react (rɪˈækt)
vb (foll by against)
1.  (intr; foll by to, upon etc) (of a person or thing) to act in response to another person, a stimulus, etc, or (of two people or things) to act together in a certain way
2.  to act in an opposing or contrary manner
3.  (intr) physics to exert an equal force in the opposite direction to an acting force
4.  chem to undergo or cause to undergo a chemical reaction
[C17: from Late Latin reagere, from re- + Latin agere to drive, do]

re-act (riːˈækt)
(tr) to act or perform again

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

react re·act (rē-ākt')
v. re·act·ed, re·act·ing, re·acts

  1. To act in response to a stimulus.

  2. To undergo a chemical reaction.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
Scientists typically respond and react after viruses have already spread.
Another imaging study showed a similar compensation effort might help older
  people react to social situations.
People react to designer labels as signals of underlying quality.
Some people react to stress by reaching for a few too many drinks.
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