The matter which density depends on pressure  or temperature :

\rho = \rho(p, T)

which means

non-zero compressibilitynon-zero thermal expansion


c = \frac{1}{\rho} \frac{\partial \rho}{\partial p} > 0 



\alpha = \frac{1}{\rho} \frac{\partial \rho}{\partial T} > 0



See also

Physics / Fluid (PVT) Analysis / Fluid (PVT) modelling 

[ Compressibility ] [ Thermal expansion coefficient ] [  Incompressible matter ]