An escutcheon is a general term for a decorative plate used to conceal a functioning, non-architectural item. Escutcheons are most often used in conjunction with mechanical, electrical, and plumbing components and fixtures where a pipe, tube, or conduit passes through a wall [or other material] surface. The escutcheon is used to bridge the gap between the outside diameter of the pipe and the inside diameter of the opening in said surface.

Escutcheon can also refer to an item of door furniture

Door furniture

Door furniture or Door hardware refers to any of the items that are attached to a door or a drawer to enhance its functionality or appearance....

. In this case, it is an architectural item that surrounds a keyhole

Keyway

A keyway is the shaped channel in a lock cylinder into which the key slides to gain access to the lock tumblers. Lock keyway shapes vary widely with lock manufacturer, and many manufacturers have a number of unique profiles requiring a specifically milled key blank to engage the lock's...

 or lock cylinder

Cylinder lock

A cylinder lock is a lock constructed with a cylinder that a locksmith can easily unscrew to facilitate rekeying. The cylinder may contain any of a variety of locking mechanisms, including the pin tumbler lock, the wafer tumbler lock and the disc tumbler lock....

. Escutcheons help to protect a lock cylinder from drilling or snapping, and the surrounding area from wear it also prevents any bugs or dust getting into the house/room.