partial isotherm (or individual isotherm)

in surface chemistry
The function relating, at constant temperature and pressure, the amount of a particular component in the interfacial layer per unit area (or per unit mass of adsorbent ) with its mole fraction (or concentration) in the liquid phase. This function can be evaluated only when the location and the thickness of the interfacial layer have been defined.
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 594