  We reconsider the thermodynamics of AdS black holes in the context of
gauge-gravity duality. In this new setting where both the cosmological constant
$\Lambda$ and the gravitational Newton constant $G$ are varied in the bulk, we
rewrite the first law in a new form containing both $\Lambda$ (associated with
thermodynamic pressure) and the central charge $C$ of the dual CFT theory and
their conjugate variables. We obtain a novel thermodynamic volume, in turn
leading to a new understanding of the Van der Waals behavior of the charged AdS
black holes, in which phase changes are governed by the degrees of freedom in
the CFT. Compared to the "old" $P-V$ criticality, this new criticality is
"universal" (independent of the bulk pressure) and directly relates to the
thermodynamics of the dual field theory and its central charge.
