In the context of theories with a first order phase transition, we propose a general covariant description of coexisting phases separated by domain walls using an additional order parameter-like degree of freedom. In the case of a holographic dual of confining and de-confining phases, the resulting model extends hydrodynamics and has a simple formulation in terms of an action and the corresponding energy-momentum tensor. The proposed
description leads to simple analytic profiles of domain walls, including the surface tension density, which agree nicely with holographic numerical solutions. We show that for such systems, the domain wall (or bubble wall) velocity can be expressed in a simple way in terms of a perfect fluid hydro-
dynamic formula, and thus in terms of the equation of state. We test the predictions for various holographic domain walls. We expect this result to
hold also for other systems.