The range over which a control valve can control. It is the ratio of the maximum to minimum controllable flow coefficients. This is also called turndown although technically it is not the same thing.
There are two types of rangeablility - Inherent and installed
inherent rangeablility is a property of the valve alone and may be defined as the range of flow coefficients between which the gain of the valve does not deviate from a specified gain by some stated tolerance limit.
Installed rangeablility is the range within which the deviation from a desired installed characteristic does not exceed some state tolerance limit.