What is defined as the greatest root-mean-square (effective) difference of potential between any two conductors of a circuit?

The greatest root-mean-square (effective) difference of potential between any two conductors in a circuit is referred to as voltage. Voltage, often measured in volts, represents the electrical potential difference that drives the movement of charge (current) through a circuit. This potential difference is crucial because it indicates the ability of the circuit to perform work, such as lighting a bulb or powering a motor.

In practical terms, voltage can be thought of as the force that pushes electric charges through a conductor. It's the measure that determines how much energy per unit of charge is available to move through the circuit. Voltage is fundamental in understanding electrical systems because without it, there would be no movement of charge, and consequently, no electric current would flow.

The other concepts—current, resistance, and power—are related to voltage but do not define the difference in potential itself. Current refers to the flow of electric charge, resistance refers to the opposition to that flow, and power is the rate at which energy is transferred or converted in a circuit. Together with voltage, these concepts are interrelated through Ohm's Law and the principles of electrical energy, but the specific definition of the effective difference in potential unequivocally points to voltage.