Voltage referenced to ground

Question

What does that mean? (Trying to understand my homework here) Thanks.

Answer 1

A voltage is measured between two points, measuring one point is meaningless.

So this means the voltage is measured with one lead on ground and the other at the point being measured.

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Answer 2

Bill has it right that you must measure a voltage potential between two points.

In standard multi-phase power distribution one of these points is always the ground for safety reasons.

Answer 3

Voltage referenced to ground means absolute value.  Example: wire a= +120V, wire b= -120V.  Voltage referenced to ground for both wires is 120V.  Voltage difference between wires a and b is 240V.