An escutcheon (/ɪˈskʌtʃən/ ih-SKUTCH-ən) 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.
An escutcheon can also refer to an item of door furniture.
In this case, it is an architectural item that surrounds a keyhole or lock cylinder, and is often part of a lockset.
Escutcheons help to protect a lock cylinder from being drilled out or snapped, and to protect the surrounding area from damage and wear from the end of the key when it misses the keyhole.