1. The work done by moving a unit positive charge from a specified location; sometimes an infinite distance, sometimes from earth's surface, to the specific point in the electric field.

Similarly, a magnetic potential exists at every point of a magnetic field, measured by the work than is needed to move a unit magnetic pole from one point in the field to another point.

2. The potential measured by the energy of a unit positive charge at a point expressed relative to an equipotential surface that has zero potential, generally the surface of the earth.
3. The work which must be done against electric forces to bring a unit charge from a reference point to the point in question.

The reference point is located at an infinite distance, or, for practical purposes, at the surface of the earth or some other large conductor.