technically it's defined as 0.01°C being the triple point of water. And since the solid-liquid boundary in the phase diagram[0] is relatively stable across wide

technically it's defined as 0.01°C being the triple point of water. And since the solid-liquid boundary in the phase diagram[0] is relatively stable across wide pressure ranges that means that 0°C being the melting point is not just true at a single pressure value. [0] https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg https://upload.wikimedia.org/wikipedia/commons/0/08/Phase_di...