Surface Resistance
The ratio of DC voltage to the current flowing between two electrodes of specified configuration that are in contact with the same side of a material. Expressed in ohms.

Surface Resistivity
For electic current flowing across a surface, the ratio of DC voltage drop per unit lenght to the surface current per width. In effect, the surface resistivity is the resistance between two opposite sides of a square and is independent of the sizes of the square of its dimensional units. It is expressed in ohms/square or simply ohms.