Source : Webster's Revised Unabridged Dictionary (1913)

Annunciator \An*nun"ci*a`tor\, n. [L. annuntiator.]
   1. One who announces. Specifically: An officer in the church
      of Constantinople, whose business it was to inform the
      people of the festivals to be celebrated.

   2. An indicator (as in a hotel) which designates the room
      where attendance is wanted.

Source : WordNet®

annunciator
     n : an indicator that announces which electrical circuit has
         been active (as on a telephone switchboard)