Pressure at which a pure liquid can exist in equilibrium with its vapour at a specified temperature. Fluids at temperatures greater than their specified (critical) temperature will exist as a vapour with no distinction between gas and liquid phases. This is a single phase liquid. At less than the critical temperature, the two fluid phases can coexist; the denser fluid phase exists as a liquid and the less dense phase as a vapour. At the critical temperature, the liquid phase is stable at pressures exceeding the vapour pressure and the gas phase is stable at pressures less than the vapour pressure. For a fluid to be liquid its surface pressure must be equal to or greater than the vapour pressure at the prevailing temperature. Higher pressures are needed to maintain volatile materials in their liquid states. The surface pressure of a liquid must be greater than its vapour pressure to maintain satisfactory operation of a centrifugal pump.