1. A thin layer of organic material covering all or part of the exposed surface of a tooth. It contains dissolved food (mostly sugar) and bacteria. The bacteria in plaque metabolize the sugar and produce acid, which eats into the surface of the enamel of the tooth and eventually causes tooth decay (dental caries).

2. A clear area in a bacterial culture grown on an agar plate due to lysis of the bacteria by a bacteriophage.

3. A patch or differentiated zone on the surface of the skin or other organ.

4. A fatty deposit that causes thickening of an arterial wall. See atherosclerosis.