Answer:
The point where all three phases coexist on a P-T diagram is called triple point.
Step-by-step explanation:
A phase diagram of a substance is a graphical representation of the transition of its physical state under various temperature and pressure conditions.
A typical phase diagram usually consists of:
(A) Fusion curve - The curve that shows that the solid and the liquid state in in equilibrium with each other.
(B) Vaporization curve - The curve that shows that the liquid and the gaseous state in in equilibrium with each other.
(C) Sublimation curve - The curve that shows that the solid and the gaseous state in in equilibrium with each other.
(D) Triple point - This is a point in the phase diagram in which all the three state exists in equilibrium.
For example, Considering the phase diagram of water, The triple point occurs at a pressure of 0.6 kPa and 0.01⁰C.
The phase diagram is shown in image below: