annunciator

Noun

 * 1) Anything that announces something.
 * 2) An officer in the church of Constantinople, whose business it was to inform the people of the festivals to be celebrated.
 * 3) A signalling device that shows which of several electrical circuits is active, especially such a device in a telephone switchboard.
 * 4)  A buzzer in a signal box that sounds when a train activates a treadle positioned on the track, and thus provides a warning or announcement of a nearby train.