  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 predicted phase transition. Lattice results of the energy
density, the pressure and the interaction measure of pure SU(3) gauge theory
are excellently reproduced. We find a relationship between the thermal energy
density of the Yang-Mills vacuum and the chromomagnetic condensate <B^2>_T.
Finally, an extension to QCD with dynamical quarks is discussed. Good agreement
with lattice data for 2, 2+1 and 3 flavour QCD is obtained. We also present the
QCD equation of state for realistic quark masses.
