From the basic conservation equations in two-phase hydrodynamics, an expression for the pressure gradient has been derived. A key quantity in the denominator of that expression is recognized as the ratio of the actual-to-sonic two-phase mass fluxes. This dimension-less ratio is seen as a generalization of the Mach number. The expression for the sonic mass flux is shown to be an equation-of-state depending only on local properties of the fluid. The conditions that make the dimension-less ratio equal to 1 are shown to correspond to the critical conditions.