Resistance and resistivity

Resistance is defined as the property of a substance due to which it opposes (or resists) the flow of electricity (electrons) through it. The unit of resistance is ohm (Ω).
A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals.
Laws of Resistance
The resistance R offered by a conductor depends on the following factors.

1. It varies directly with its length l.
2. It varies inversely as the cross section A of the conductor
3. It depends on the nature of the material
4. It also depends on the temperature of the conductor

Neglecting the temperature effect, we get
R α l/A
R= ρ l/A
where ρ is a constant depending on the nature of the material of the conductor an is known as its specific resistance or resistivity.
ρ = RA/l.
Resistivity
The resistivity of a material may be defined as the resistance between the opposite faces of a meter cube of that material . Its unit is Ωm.