  A new scaling variable is introduced in terms of which nuclear shadowing in
deep-inelastic scattering is universal, i.e. independent of $A$, $Q^2$ and $x$.
This variable can be interpreted as a measure of the number of gluons probed by
the hadronic fluctuations of a virtual photon during their lifetime. The
shadowing correction grows at small $x$ substantially less steeply than is
suggested by the eikonal approximation. This results from the fact that
shadowing is dominated by soft rather than hard interactions.
