CFD Online Discussion Forums - What is "concentration susceptibility"?

Hello everyone,
This phrase makes me confused when I read papers on Soret and Dufour effect.

$u\frac{{\partial T}}{{\partial x}} + v\frac{{\partial T}}{{\partial y}} = \frac{k}{{\rho {c_p}}}\frac{{{\partial ^2}T}}{{\partial y}} + \frac{{{D_m}{k_T}}}{{{C_s}{c_p}}}\frac{{{\partial ^2}C}}{{\partial {y^2}}}$

where

is concentration susceptibility.

is mass diffusivity,

is specific heat at constant pressure,

is thermal diffusion ratio.
$C,C_{\infty} ,C_w$ is concentration.
it is also used to calculate Dufour number:
${D_f} = \frac{{{D_m}{k_T}({C_w} - {C_\infty })}}{{{C_s}{c_p}{\alpha _m}({T_w} - {T_\infty })}}$
${\alpha _m} = \frac{k}{{\rho {c_p}}}$
But how to define "concentration susceptibility"?I used Google search and did not get what I need.
Since I am not an English native speaker, I just can not understand what "susceptibility" means here.

Thanks.