The distance from the electrode where the ratio $$\frac{c-c_{\text{e}}}{c_0-c_{\text{e}}}$$ reaches a given value. ($c_{\text{e}}$ is the interfacial concentration, $c$ is the concentration, $c_0$ is the value in the bulk solution.) If this ratio is selected as, say, 0.99 the corresponding thickness of the diffusion layer is denoted by ${\delta }_{0.99}$.