triple point

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The temperature and pressure at which a substance can exist in equilibrium in the liquid, solid, and gaseous states. The triple point of pure water is at 0.01 degrees Celsius and 4.58 millimeters of mercury and is used to calibrate thermometers.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The unique temperature and pressure at which the solid, liquid and gas phases of a substance are all in equilibrium.