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Triple point

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The triple point of water or any substance is the combination of temperature and pressure at which it exists in its solid, liquid, and gaseous state all at once, in equilibrium.

The temperature of the triple point of water is a commonly used physical constant which is used to define temperature scales and calibrate temperature-measuring systems.

By definition, the temperature of the triple point of water is 273.16 Kelvin, or 0.01 °C. One degree Celsius is defined as 1/273.16th part of the triple point of water. This fixes the temperature scale as defined in the ITS 90, which is the international temperature scale of 1990. This scale is developed as a practical approximation of the thermodynamical temperature scale.