partition constant, K D o

https://dx.doi.org/10.1351/goldbook.P04438
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:
K D o A = a A,org a A,aq
Its value should not vary with composition but depends on the choice of standard states and on the temperature (and eventually the pressure).
Source:
PAC, 1993, 65, 2373 (Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)) on page 2385