The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. An indicator (as in a hotel) which designates the room
      where attendance is wanted.
      [1913 Webster]

WordNet (r) 3.0 (2006):

annunciator
    n 1: an indicator that announces which electrical circuit has
         been active (as on a telephone switchboard)