Question

what is a triple point of water? why it is called triple point ?

Answer 1

triple point. The temperature and pressure at which a substance can exist in equilibrium in the liquid, solid, and gaseous states. The triple point of pure water is at 0.01°C (273.16K, 32.01°F) and 4.58 mm (611.2Pa) of mercury and is used to calibrate thermometers

Answer 2

The "triple point of water" is the point where one is interested in all "three phases" of water at a certain "pressure and temperature". The three phases includes ice, water, and vapour. By slightly adjusting either pressure or temperature, one of the "three phases" will change the phase depending on the changes of its environment.

It is called "triple point" because at a particular temperature and pressure for particular substances, solid, liquid, and vapour phases coexist in equilibrium. It is a specific case of equilibrium of the thermodynamic phase.