Plaque-forming unit
A plaque-forming unit (PFU) is a measure used in virology to describe the number of virus particles capable of forming plaques per unit volume. It is a proxy measurement rather than a measurement of the absolute quantity of particles: viral particles that are defective or which fail to infect their target cell will not produce a plaque and thus will not be counted.
For example, a solution of tick-borne encephalitis virus with a concentration of 1,000 PFU/μl indicates that 1 μl of the solution contains enough virus particles to produce 1000 infectious plaques in a cell mono-layer, but no inference can be made about the relationship of pfu to number of virus particles. The concept of plaque-forming units of virus is equivalent to the concept of colony-forming units of bacteria.