Triple Point
The unique temperature and pressure at which the solid, liquid and vapour phases of a substance coexist in equilibrium.
Unit
K, Pa
In depth
Because it is fixed by nature and independent of any apparatus, the triple point makes an excellent temperature standard, and water's defined the kelvin until 2019. Below the triple point pressure a substance cannot exist as a liquid at all and sublimes instead.
Examples in the real world
Water's triple point is 273.16 K and 0.61 kPa; CO₂'s is at 5.1 atm, which is why dry ice sublimes at 1 atm.