The mathematical expression of a property of a point in space, measuring
the tendency which existing electric forces would have to drive an
electrified unit particle away from or prevent its approach to the point
in question, according to whether the point was situated at or was at a
distance from the point in question.

Potential is not the power of doing work, although, as it is expressed
always with reference to a unit body, it is numerically equal to the
number of ergs of work which must be done in order to bring a positive
unit of electricity from a region where there is no electric
force–which is a region at an infinite distance from all electrified
bodies–up to the point in question. This includes the assumption that
there is no alteration in the general distribution of electricity on
neighboring bodies. (Daniell.)

In practice the earth is arbitrarily taken as of zero electric potential.