Webster's English Dictionary

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reactor

re.ac.tor \re--'ak-t*r\ n 1: one that reacts; specif : a subject reacting 
   positively to a for eign substance 2: a device (as a coil, winding, or 
   conductor of small resistance) used to  introduce reactance into an 
   alternating-current circuit 3a: a vat for an industrial chemical reaction  
   3b: an apparatus in which a chain reaction of fissionable material is initi 
   ated and controlled