Empirically-determined quantity defined by the equation $$\mathcal{Q}=(1/Ac)\mathrm{\Delta }Q/\mathrm{\Delta }(t^{1/2})$$ where $A$ is the electrode surface area, $c$ is the bulk concentration of the electroactive substance, $Q$ is the charge passed, and $\mathrm{\Delta }Q/\mathrm{\Delta }(t^{1/2})$ is the slope of a plot of $Q$ against $t^{1/2}$.