plaque

In microbiology:

A clear region in the top agar indicating lysis of bacterial cells by the action of bacteriophage in the medium. Plaques will contain a population of free phage that is descended from a single common ancestor, responsible for the original infection of a host cell. This property makes the plaque a useful experimental unit in microbial genetics. Like bacterial colonies, plaques may vary in morphology; diameter can be taken as an indicator of fitness.

In dentistry, a soft, sticky, whitish matlike film attached to tooth surfaces, formed largely by the growth of bacteria that colonize the teeth. What the hygienist, armed with various pointy instruments, is determined to forcibly remove. Excessive plaque build-up will lead to halitosis and an unpleasant, furry appearance to the teeth.

Plaque (?), n. [F. Cf. Plack, and see Placard.]

Any flat, thin piece of metal, clay, ivory, or the like, used for ornament, or for painting pictures upon, as a slab, plate, dish, or the like, hung upon a wall; also, a smaller decoration worn on the person, as a brooch.

© Webster 1913.