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.