"one that reacts," 1835, agent noun in Latin form from react. By 1915 in electricity as "coil or other piece of equipment which provides reactance in a circuit;" the nuclear sense is attested from 1945.
Entries linking to reactor
react v.
1640s, "to exert, as a thing acted upon, an opposite action upon the agent," from re- "back" + act (v.). Related: Reacted; reacting (1610s). For sense development, see reaction. The verb meaning "to perform again, do a second time" (often written re-act and given full pronunciation of the prefix to distinguish it from react) is from 1650s, from the "again" sense in re-.