Defined by the equation: \[\delta = \frac{D}{k_{\text{d}}}=\frac{n\ F\ D\ c\ A}{l_{i}}\] where D is the diffusion coefficient, kd is the heterogeneous diffusion rate constant, F is the Faraday constant, c is the concentration, A is the geometric area of the electrode, li is the limiting current and n is the charge number of the cell reaction.