  We show that holographic models of QCD predict the presence of a Chern-Simons
coupling between vector and axial-vector mesons at finite baryon density. In
the AdS/CFT dictionary, the coefficient of this coupling is proportional to the
baryon number density, and is fixed uniquely in the five-dimensional
holographic dual by anomalies in the flavor currents. For the lightest mesons,
the coupling mixes transverse $\rho$ and $a_1$ polarization states. At
sufficiently large baryon number densities, it produces an instability, which
causes the $\rho$ and $a_1$ mesons to condense in a state breaking both
rotational and translational invariance.
