The ratio of activity of a given species A in the extract to its activity in the other phase with which it is in equilibrium, thus:
Its value should not vary with composition but depends on the choice of standard states and on the temperature (and eventually the pressure).
PAC, 1993, 65, 2373
(Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993))
on page 2385
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook