  The product of the gluon dressing function and the square of the ghost
dressing function in the Landau gauge can be regarded to represent, apart from
the inverse power corrections $1/Q^{2 n}$, a nonperturbative generalization
$A(Q^2)$ of the perturbative QCD running coupling $a(Q^2)$ ($\equiv
\alpha_s(Q^2)/\pi$). Recent large volume lattice calculations for these
dressing functions strongly indicate that such a generalized coupling goes to
zero as $A(Q^2) \sim Q^2$ when the squared momenta $Q^2$ go to zero ($Q^2 \ll 1
\ {\rm GeV}^2$). We construct such a QCD coupling $A(Q^2)$ which fulfills also
various other physically motivated conditions. At high momenta it becomes the
underlying perturbative coupling $a(Q^2)$ to a very high precision. And at
intermediately low momenta $Q^2 \sim 1 \ {\rm GeV}^2$ it gives results
consistent with the data of the semihadronic $\tau$ lepton decays as measured
by OPAL and ALEPH. The coupling is constructed in a dispersive way, ensuring as
a byproduct the holomorphic behavior of $A(Q^2)$ in the complex $Q^2$-plane
which reflects the holomorphic behavior of the spacelike QCD observables.
Application of the Borel sum rules to $\tau$-decay V + A spectral functions
allows us to obtain values for the gluon (dimension-4) condensate and the
dimension-6 condensate, which reproduce the measured OPAL and ALEPH data to a
significantly better precision than the perturbative MSbar coupling (+OPE)
approach. The comparison with the experimental V-channel Adler function,
related with the $e^+ e^- \to$ hadrons ratio, at low $Q^2 \sim 1 \ {\rm
GeV}^2$, also gives results considerably better than with the usual MSbar
pQCD+OPE approach.
