The triple point is the temperature and pressure at which three phases (gas, liquid, and solid) of a particular substance may coexist in thermodynamic equilibrium. 

For water the triple point temperature is 273.16 K (0.01 C) and the triple point pressure is 611.73 Pa.

Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.

See also Equations of State.

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