A man pressing the buzzer on an intercom.
buzzer definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
an electrical device that makes a buzzing sound as a signal
buzzer definition by American Heritage Dictionary
An electric signaling device, such as a doorbell, that makes a buzzing sound.