thickness of diffusion layer

in electrochemistry
https://doi.org/10.1351/goldbook.T06342
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 @[email protected], \(c\) 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 @[email protected] layer is denoted by \(\delta_{0.99}\).
See also:
diffusion layer
Source:
PAC, 1981, 53, 1827. (Nomenclature for transport phenomena in electrolytic systems) on page 1837 [Terms] [Paper]