Voltage
Technically, the voltage is the difference in electric potential between two points and is always measured between two points.  e.g. between the positive and negative ends of a battery, between a wire and the ground, or between a wire or a point of a circuit and a point in another part of the circuit.  In everyday use with household electricity in the U.S. the voltage is most often 120V. This voltage is measured from the electric wire to the ground.