The Collaborative International Dictionary of English v.0.48:

Commutator \Com"mu*ta`tor\, n. (Elec.)
   A piece of apparatus used for reversing the direction of an
   electrical current; an attachment to certain electrical
   machines, by means of which alternating currents are made to
   be continuous or to have the same direction. It may be
   attached to the end of the spindle of an electric motor,
   where a brush is in contact sequentially with the parts of
   the spindle that conduct current to the different windings of
   the motor.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

commutator
    n 1: switch for reversing the direction of an electric current