https://doi.org/10.1351/goldbook.09131
Empirically-determined quantity defined by the equation \[{\cal Q} = (1/Ac)\Delta Q/\Delta (t^{1/2})\] where \(A\) is the electrode surface area, \(c\) is the bulk concentration of the electroactive substance, \(Q\) is the charge passed, and \(\Delta Q/\Delta(t^{1/2})\) is the slope of a plot of \(Q\) against \(t^{1/2}\).
Note: The SI unit of \(\cal Q\) is \(\pu{A s^{1/2} mol-1 m}\).