  A hydrodynamic theory of screening in a generic electron gas of arbitrary
dimensionality is given that encompasses all previously studied cases and
clarifies the predictions of the many-body approach. We find that
long-wavelength plasma oscillations are classical phenomena with
quantum-mechanical effects playing no explicit role. The character of the
oscillations is solely dictated by the dimensionality of the electron system
and its equation of state in the neutral limit. Materials whose excitations are
described by the Dirac dispersion law -- such as doped graphene or a Weyl
semimetal -- are no exception to this rule.
