The Collaborative International Dictionary of English v.0.48:

Reactor \Re*act"or\, n.
   1. (Elec.) A choking coil.
      [Webster 1913 Suppl.]

   2. A nuclear reactor.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Choking coil \Choking coil\ (Elec.)
   A coil of small resistance and large inductance, used in an
   alternating-current circuit to impede or throttle the
   current, or to change its phase; -- called also reactance
   coil or reactor, these terms being now preferred in
   engineering usage.
   [Webster 1913 Suppl.] Choky

WordNet (r) 3.0 (2006):

reactor
    n 1: an electrical device used to introduce reactance into a
         circuit
    2: (physics) any of several kinds of apparatus that maintain and
       control a nuclear reaction for the production of energy or
       artificial elements [syn: nuclear reactor, reactor]