From Glossary of Meteorology
(Or triple point.) The thermodynamic state at which three phases of a substance exist in equilibrium.
For water substance, the triple state occurs at es = 0.611 kPa, and T = 273.16 K, where es is the saturation vapor pressure of the water vapor and T the Kelvin temperature. At this state the specific volumes of the water vapor, water, and ice are, respectively,