  Relativistic fluids are Lorentz invariant, and a non-relativistic limit of
such fluids leads to the well-known Navier-Stokes equation. However, for fluids
moving with respect to a reference system, or in critical systems with generic
dynamical exponent z, the assumption of Lorentz invariance (or its
non-relativistic version) does not hold. We are thus led to consider the most
general fluid assuming only homogeneity and isotropy and study its
hydrodynamics and transport behaviour. Remarkably, such systems have not been
treated in full generality in the literature so far. Here we study these
equations at the linearized level. We find new expressions for the speed of
sound, corrections to the Navier-Stokes equation and determine all dissipative
and non-dissipative first order transport coefficients. Dispersion relations
for the sound, shear and diffusion modes are determined to second order in
momenta. In the presence of a scaling symmetry with dynamical exponent z, we
show that the sound attenuation constant depends on both shear viscosity and
thermal conductivity.
