The Collaborative International Dictionary of English v.0.48:

doorbell \doorbell\ n.
   a bell or other sounding device, actuated by a push button at
   an outer door; the push button activating the bell; alos, the
   ringing of such a bell; as, I was in the shower and didn't
   hear the doorbell.

   Syn: bell, buzzer.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

doorbell
    n 1: a push button at an outer door that gives a ringing or
         buzzing signal when pushed [syn: doorbell, bell,
         buzzer]