A measure of the quantity of individual infectious particles (e.g. virus particles) based on the number of plaque formed per unit volume.
Theoretically, the plaque-forming unit includes only the infectious virus particles since a virus particle failing to infect a host cell will not be able to produce a plaque, hence, will not be counted. PFU rules out possible multiple-hit phenomena and include only the particles capable of infecting cells on their own. Thus, one PFU means one lytic event (or one infectious virus particle).
Compare: colony-forming unit (CFU).