  The viscous motion of an interface driven by an ac external field of
frequency omega_0 in a random medium is considered here for the first time. The
velocity exhibits a smeared depinning transition showing a double hysteresis
which is absent in the adiabatic case omega_0 --> 0. Using scaling arguments
and an approximate renormalization group calculation we explain the main
characteristics of the hysteresis loop. In the low frequency limit these can be
expressed in terms of the depinning threshold and the critical exponents of the
adiabatic case.
