n 1  :  one that reacts: as  a  :  a chemical reagent  b  :  an individual reacting to a stimulus  c  :  an individual reacting positively to a foreign substance (as in a test for disease) <s to tuberculin>   2 a  :  a piece of equipment in which a chemical reaction and esp. an industrial chemical reaction is carried out  b  :  a device for the controlled release of nuclear energy (as for producing heat) .
 
 
 
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