Resistance may be defined as that property of a substance which opposes (or restricts) the flow of current (or electrons) through it. The practical as well as MKS (or SI) unit of ohm (O). 1 ohm is defined as that resistance between two points of a conductor which produces a current of one ampere. When a potential difference of 1 volt is applied to it.
If I is the current flowing through a conductor of resistance R across which a potential difference V is applied then according to ohm's law.
Where V is in volts, R is in the ohms and I is in amperes.
Resistance of a material is:
(a) Directly proportional to its length, l R α l
(b) Inversely proportional to its area of cross section a, Rα1/a
Combining above two facts me have R α l/a
Where ρ(rho) is a constant depending upon the nature of the material and is known as the specific resistance or resistivity of the material. Putting l=1 and a=1 expression we have R=ρ. Hence resistance of a material of unit length having unit cross-sectional area is defined as the resistivity.