triple point

n. (Physics) temperature and pressure under which three phases (solid, liquid and gas) exist at the same time

In physics and chemistry, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. For example, the triple point temperature of mercury is at −38.8344 °C, at a pressure of 0.2 mPa. The triple point of water is used to define the kelvin, the SI base unit of thermodynamic temperature. The number given for the temperature of the triple point of water is an exact definition rather than a measured quantity.
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The intersection point between two boundaries (dry line , outflow boundary , cold front , etc.), often a focus for thunderstorm development.
Triple point also may refer to a point on the gust front of a supercell , where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft , and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).

The temperature and pressure at which the solid, liquid, and gaseous forms of a substance are at equilibrium .