  We compute the finite-temperature and matter density corrections to the
S-parameter at the one loop level. At non-zero temperature T and matter density
Lorentz symmetry breaks and therefore we suggest a suitable generalization of
the S-parameter. By computing the plasma correction, we discover a reduction of
the S-parameter in the physically relevant region of small external momenta for
any non-zero chemical potential and T. In particular, the S-parameter vanishes
at small m/T, where m is the mass of the fermions, due to the finite extent of
the temporal direction. Our results are directly applicable to the
determination of the S-parameter via first principle lattice simulations
performed with anti-periodic boundary conditions in the temporal direction.
