  The Dark Sector is described by an additional barotropic fluid which evolves
adiabatically during the universe's history and whose adiabatic exponent
$\gamma$ is derived from the standard definitions of specific heats. Although
in general $\gamma$ is a function of the redshift, the Hubble parameter and its
derivatives, we find that our assumptions lead necessarily to solutions with
$\gamma = $ constant in a FLRW universe. The adiabatic fluid acts effectively
as the sum of two distinct components, one evolving like non-relativistic
matter and the other depending on the value of the adiabatic index. This makes
the model particularly interesting as a way of simultaneously explaining the
nature of both Dark Energy and Dark Matter, at least at the level of the
background cosmology. The $\Lambda$CDM model is included in this family of
theories when $\gamma = 0$. We fit our model to SNIa, $H(z)$ and BAO data,
discussing the model selection criteria. The implications for the
early-universe and the growth of small perturbations in this model are also
discussed.
