doorbell

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A bell, chime, or buzzer outside a door that is rung to announce the presence of a visitor or caller.

from The Century Dictionary.

• noun A bell at a door, or connected with a handle or knob exposed outside a door, for the purpose of giving notice when one desires admittance.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device on or adjacent to an outer door for announcing one's presence. It can be mechanical, directly sounding a bell, or a button that electrically sounds a chime or buzzer inside the building.
• noun A button that actives an electric doorbell.
• verb intransitive To ring many doorbells in an effort to contact people and thereby spread information or solicit.
• verb transitive To ring many doorbells of (target people or an area) in an effort to contact people and thereby spread information or solicit.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a push button at an outer door that gives a ringing or buzzing signal when pushed