Of component
,
defined by the equation
where
,
and
are, respectively, the total Gibbs
surface concentration and the total concentrations in the bulk
phases α and β:
The reduced adsorption is invariant to the location of the Gibbs surface. Alternatively, the reduced adsorption may be regarded as the Gibbs surface concentration of
when the Gibbs surface is chosen so that
is zero, i.e. the Gibbs surface is chosen so that the reference system has not only the same volume, but also contains
the same total amount of substance() as the real system.

Source:

PAC, 1972, *31*, 577*
(Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix
II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry)
* on page 591