A flat plate, slab, or disk that is ornamented or engraved for mounting, as on a wall for decoration or on a monument for information. A small pin or brooch worn as an ornament or a badge of membership. Pathology A small disk-shaped formation or growth; a patch. A deposit of fatty material on the inner lining of an arterial wall, characteristic of atherosclerosis. A scaly patch formed on the skin by psoriasis. A film of mucus and bacteria on a tooth surface. A clear, often round patch of lysed cells in an otherwise opaque layer of a bacteria or cell culture.