Triple Point
Definition
The unique temperature and pressure where solid, liquid, and gas phases of a substance coexist in equilibrium.
Technical definition
A fixed point on the phase diagram where all three phase boundaries meet. Water's triple point (273.16 K, 611.657 Pa) was historically used to define the kelvin. Each pure substance has exactly one.
Example
Water's triple point at 0.01°C and 0.006 atm: in a sealed cell at these conditions, ice, liquid water, and vapor sit together indefinitely.
Common confusion
The triple point is not the same as the melting or boiling point — those depend on pressure. The triple point is a single (T, P) coordinate unique to each substance, fixed by nature.