Determination of phage titers as indicated by localized lysis of bacterial lawns.
That is, a measure of virus viability that is scored in terms of visible host absence when grown in or on solid or semi-solid media.
Plaque counts are determinations of plaque-forming units and are subject to inefficiencies described in terms of efficiencies of plating. Generally, though, a plaque count is simply the number of plaques that are formed upon phage plating and, given knowledge of both volume and degrees of dilution, can be used to calculate titers especially of plaque-forming units found within a culture or phage stock.
Note that not only do plaque counts determine numbers of plaque-forming units rather than numbers of actual phage particles (see, for example, absolute efficiency of plating), but plaque counts cannot necessarily be equated with numbers of otherwise viable virion particles. See efficiency of plating as well as efficiency of center of infection for additional discussion.
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