  Various definitions of the symmetry energy are introduced for nuclei, dilute
nuclear matter below saturation density and stellar matter, which is found in
compact stars or core-collapse supernovae. The resulting differences are
exemplified by calculations in a theoretical approach based on a generalized
relativistic density functional for dense matter. It contains nucleonic
clusters as explicit degrees of freedom with medium dependent properties that
are derived for light clusters from a quantum statistical approach. With such a
model the dissolution of clusters at high densities can be described. The
effects of the liquid-gas phase transition in nuclear matter and of cluster
formation in stellar matter on the density dependence of the symmetry energy
are studied for different temperatures. It is observed that correlations and
the formation of inhomogeneous matter at low densities and temperatures causes
an increase of the symmetry energy as compared to calculations assuming a
uniform uncorrelated spatial distribution of constituent baryons and leptons.
