Webster's English Dictionary

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reaction

re.ac.tion \re--'ak-sh*n\ \-shn*l, -sh*n-*l\ \-e-\ n 1a: the act or process 
   or an instance of reacting  1b: tendency toward a former esp. outmoded 
   political or social order or pol icy 2: bodily response to or activity 
   aroused by a stimulus :  2a: an action induced by vital resistance to 
   another action; esp :  the result characteristically evoked in tissues by a 
   foreign substance and used to determine specific sensitivities or presence 
   of infection 2b: depression or exhaustion due to excessive exertion or 
   stimulation  2c: heightened activity and overaction succeeding depression 
   or shock  2d: a mental or emotional disorder forming an individual's 
   response to his  life situation 3: the force that a body subjected to the 
   action of a force from another bo dy exerts in the opposite direction 4a1: 
   chemical transformation or change  4a2: the state resulting from such a 
   reaction  4b: a process involving change in atomic nuclei  - re.ac.tion.al 
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