"The greatest root-mean-square difference of potential between any two conductors of the circuit concerned" is known as what?

The term described as "the greatest root-mean-square difference of potential between any two conductors of the circuit concerned" refers specifically to voltage. Voltage is the measure of electric potential difference between two points in a circuit and is often expressed in volts.

The root-mean-square (RMS) value is particularly relevant in alternating current (AC) systems where voltage fluctuates over time. The RMS voltage provides a way to quantify this fluctuating voltage in a manner that reflects its ability to do work or provide power, much like a steady DC voltage would.

Understanding voltage as the potential difference helps in analyzing electrical circuits and components as it directly affects the flow of current according to Ohm's Law (V = IR). Hence, when discussing the variations of electric potential difference, voltage is indeed the correct term that fits the definition provided in the question.