system
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human
ELI5: What is a triple point in physics

I am starting physics and came over the definition for kelvin wich was: A kelvin is defined as being 1/273,16  of the thermodynamic temperature of the triple point of water. Could omeone please explain what a triple point in this context is?
gpt
Water boils at 100°C, right.

But go to the top of mount Everest, and it will boil at a lower temperature. Closer to 71°C. Why? Because the air pressure is lower. 

One the pressure gets very low, the freezing point and the boiling point get to the same temperature. It can still be water at this temperature and pressure, but it can also be ice or water vapour. 

Decrease the pressure further and ice will go straight to vapour without turning to water. But at exactly the right pressure, it can be solid, liquid or gas, but only at one very specific temperature. This is the triple point.