The phenomenon of polarization is best explained in terms of cathodic and anodic reactions when a metal system such as   Cu²⁺ + 2e Cu  is at equilibrium, the rate of forward anodic reaction is equal to the rate of reverse cathodic reactions

Cu²⁺ + 2e Cu 

This is not net reaction and therefore there is no net current flow. The potential across the metal/solution interface is the equilibrium potential (E_equilibrium). However, if the system is displaced from equilibrium and net anodic or cathodic current flows, and the potential varies with the current. The shift of potential from the equilibrium is called over potential or polarization, h.

h = E_{applied potential} - E_{equilibrium potential}

At equilibrium potential, E_eq also called E_corr (corrosion potential), h = 0. Any departure from E_eq gives a measure of polarization.