In electrostatics and circuit theory, potential (voltage) at a single point is defined relative to a reference point (usually infinity or ground where potential is assumed to be zero).
Absolute voltage at a point is not a physically measurable quantity because only the difference in potential between two points can be measured using instruments like a voltmeter.

Resistance can be measured using an ohmmeter or bridge circuits.
Displacement current is a physical quantity that can be calculated and its effects (magnetic field) can be measured. Voltage difference (potential difference) is the standard quantity measured between two points in a circuit. Therefore, absolute voltage is not a measurable quantity.