Substances exist in solid, liquid and vapor phase – the such a called thermodynamic phases. Under normal conditions, the phases transfer from solid to liquid to gas and the other way around.
At the tripple point a substance can exist in three different thermodynamic phases in parallel (e.g. solid, liquid and gas phase).
The phase diagram shows, that the phase boundaries meet at the tripple point.
Helium is the only known substance, which has no tripple point with solid/liquid and gas phase.
All other materials can exist in these three phases at the same time.

Water (H2O), same as other substances is present at the tripple point in solid, liquid and gas phase, also called the thermodynamic equilibrium.
Below the tripple point water changes its phase directly from gas to solid, also called frost point.

Tripple Point in Injection Molding

Phase diagram with the tripple point at the intersection between the phases solid (blue), liquid (purple) and gaseous (gray).

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.