1. A visual or audible signaling device that indicates the conditions of associated circuits. Note: Early annunciators were operated by relays. Modern annunciators use electronic and fiber optic systems to display letters and numerals. 2. A visual or audible signaling device used to make announcements, post messages, or create sound signals, such as the “five o'clock whistle” or a sign made with letters or numerals that can be changed to announce train, bus, or aircraft arrivals and departures. See also circuit,  condition,  device,  display,  electronic,  fiber optic system,  letter,  lightpipe,  message,  numeral,  optical signal,  signaling,  relay,  visual,  signal,  transmission system.