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.

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.