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: $${\left(K_{\text{D}}^{\text{o}}\right)}_{\text{A}}=\frac{a_{\text{A,org}}}{a_{\text{A,aq}}}$$ Its value should not vary with composition but depends on the choice of standard states and on the temperature (and eventually the pressure).