drop indicator

drop indicator

[¦dräp ′in·də‚kād·ər]
(communications)
Indicator for signaling, consisting of a hinged flap normally held up by a catch; the catch is released by an electromagnet, allowing the flap to drop when a signal is received.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.