Change in electrical potential between two points.
A change in electrical potential between two points in an electric circuit is called a "potential difference". The "electromotive force (e.m.f)" provided by a source of energy - such as a generator or battery - is measured in volts.
One volt i one joule per coulomb and is defined as the difference in potential between two points in a conductor which, when carrying a current of one ampere, dissipates a power of one watt:
volts = watts / amperes = (joules/sec) / amperes = joules / ampere seconds = joules / coulombs