The flow coefficient of a valve is a relative measure of its efficacy at allowing fluid flow. It describes the relationship between the pressure drop across an orifice and the corresponding flow rate.
There are two standards for valve CV. The most common is the volume in US gallons of water in one minute that will through a valve with a pressure drop of 1 psi across the valve.
The other standard is for compressible gas.
Wikipedia has the formulas for the CV of valves. The CV for gas flow is most often needed for flow calculations in spudding.
 Wikipedia page on CV.