Potential difference is the difference in electric potential between two points A and B.
ΔV = V(B) - V(A)
Most accurately, it is the path integral of the electric field from point A to point B:
ΔV = ∫E(r)∙dl, path=A..B
This can be interpreted as the amount of work it takes to move a positive test charge from point A to point B through the electric field. Equivalently, this is the amount of potential energy the test charge will gain when it moves from point A to B.