  We propose a novel quasiparticle interpretation of the equation of state of
deconfined QCD at finite temperature. Using appropriate thermal masses, we
introduce a phenomenological parametrization of the onset of confinement in the
vicinity of the phase transition. Lattice results of the energy density, the
pressure and the interaction measure of pure SU(3) gauge theory are well
reproduced. A relation between the thermal energy density of the Yang-Mills
vacuum and the chromomagnetic condensate <B^2>_T is found. We also present the
two flavour QCD equation of state for realistic quark masses and apply the
model to dilepton production in ultrarelativistic heavy-ion collisions.
