The flow rate in gallons per minute of water with a specific gravity of 1 provided by a pressure differential of 1.0 psi through a fully opened control valve is a definition of:

The correct choice is Cv, which stands for flow coefficient. Cv is a measure that indicates how much flow a valve can handle at a given pressure drop. Specifically, in this context, it quantifies the flow rate through a control valve when the pressure differential across the valve is 1.0 psi, and it is used when dealing with fluids like water that have a specific gravity of 1.

This flow coefficient is essential in valve sizing and selection for control applications. It allows engineers and technicians to understand how much fluid can pass through a valve under specific conditions of pressure and specific gravity. The definition given in the question directly aligns with the concept of Cv, as it describes flow rate in gallons per minute relative to pressure differential, accurately reflecting the function of a control valve in a fluid system.