A flat, thin piece of metal or earthenware upon which a picture or design is enameled or painted: used chiefly as a wall ornament.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
A flat piece of metal on which an enamel is painted; also the enamel itself; a similar ornamental piece.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)

Usage examples:

But what's the round vertical plaque
- The Moghul by Thomas Hoover
" Wait, here's a plaque set in the wall.
- Ralestone Luck by Andre Norton
He may also inspect with profit the handicraft of a lowly mollusc which agglutinates sand- grains into a kind of plaque in the substance of which numerous eggs are deposited.
- My Tropic Isle by E J Banfield