--Not directly related to the OP--
ejef and mecha you are saying slightly different but not mutually-exclusive things.
Ejef is referring to the more fundamental and general definition of voltage. According to that voltage is proportional to the work done (energy used) when moving a charge from one point to another. If there is no movement, there is no work and you cannot define voltage. Thats why voltage is more formally referred to as voltage difference, or even more formally potential diferrence as voltage came from Volta, the guy most famous for inventing the battery and whose name was given as a unit of measure to the potential difference betwen two point. Describing it with integrals is another elegant way, but maths follows nature, not nature maths. Moving on....
Mecha is taking it to the next practical level of saying that ok, if we know that the difference between two points is so many volts, then if we set one point arbitarily as ejef said
to 0 Volts in our mind, then the other point immediately is at a voltage equal to the potential difference. Mathematically, if |DeltaV| = |Va - Vb|, then if we set Va to 0 Volts then |DeltaV| = |Vb|.
Hope this helps with the direction of the conversation.