Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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