Plaque: in bacteriology and genetics, a plaque forms in a bacteriophage infection experiment. Bacteria are grown as a lawn (i.e., they evenly cover the entire surface of a Petri dish), then injected with bacteriophage. The phage will kill and lyse bacterial cells, leaving a clear spot on the lawn. This clear spot is called a plaque. Plaques contain phage particles, and are equivalent to colonies of phage.