  Non-perturbative effects in the high-temperature phase of the electroweak
theory are characterized by a magnetic screening length. Its size influences
the range of validity of perturbation theory, and it also determines the
critical Higgs boson mass where the first-order phase transition changes to a
crossover. We propose a gauge-invariant definition of the magnetic screening
length and discuss its role in several gauge-dependent and gauge-invariant
correlation functions.
