The Collaborative International Dictionary of English v.0.48:

doorknocker \doorknocker\ n.
   a device (usually metal and ornamental) attached by a hinge
   to a door, which may be swung by hand against the door to
   make a loud knocking sound.

   Syn: knocker.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

doorknocker
    n 1: a device (usually metal and ornamental) attached by a hinge
         to a door [syn: knocker, doorknocker, rapper]