What potential difference is
The electric pressure difference the water tank stands for has a proper name.
Potential difference
The electric pressure difference set up across a conductor is called the potential difference.
The unit
The unit used for measuring potential difference is the volt.
V
The volt (V) — the unit in which potential difference is measured.
It is always between two points
Notice that a potential difference is never a property of one place on its own. It is a difference, so it is always quoted between two points — across a bulb, across a cell, between the two ends of a wire. A question that asks for "the potential difference" without naming two points has not finished asking.
Potential difference is measured in volts
Potential difference is the electric push that drives a current, it is measured in volts (V), and it always exists between two points.