  The suspicion that gravity is holographic has been supported mainly by a
variety of specific examples from string theory. In this paper, we propose that
such a holography can actually be observed in the context of Einstein's gravity
and at least a class of generalized gravitational theories, based on a definite
holographic principle where neither is the bulk space-time required to be
asymptotically AdS nor the boundary to be located at conformal infinity,
echoing Wilson's formulation of quantum field theory. After showing the general
equilibrium thermodynamics from the corresponding holographic dictionary, in
particular, we provide a rather general proof of the equality between the
entropy production on the boundary and the increase of black hole entropy in
the bulk, which can be regarded as strong support to this holographic
principle. The entropy production in the familiar holographic
superconductors/superfluids is investigated as an important example, where the
role played by the holographic renormalization is explained.
