What is the term used for a substance held at temperature and pressure where the solid, liquid, and gaseous states exist simultaneously?

The term that describes the condition under which the solid, liquid, and gaseous states of a substance coexist in equilibrium is known as the triple point. At this specific combination of temperature and pressure, all three phases of the material can exist simultaneously without any one phase dominating. This is a unique feature of each substance, with the triple point being a precise point on a phase diagram.

Understanding the triple point is important in thermodynamics and helps in defining the properties of different substances. It has practical applications, such as in the calibration of thermometers and the study of phase transitions.

The melting point refers specifically to the temperature at which a solid transitions into a liquid, and the boiling point denotes the temperature at which a liquid transitions into a gas. The critical point, on the other hand, is the temperature and pressure at which the distinction between liquid and gas phases disappears, leading to a supercritical fluid, which does not include the coexistence of solid state.