When warmed, a typical solid (such as ice) eventually will begin to melt at some temperature. A pure solid usually has a characteristic melting point. This temperature may depend on other factors, such as pressure.

Consider the situation where we have warmed some ice and it has started to melt. The accepted melting temperature (melting point) of water is 0°C. What happens if the system is carefully held exactly at that temperature? Will the system melt, or will it refreeze? The answer is that it will do both! The amount of ice remaining and the level of liquid water will stay constant as long as the system is kept at the exact melting point.

Changes have not stopped. Instead, the rate of melting has become equal to the rate of freezing. We say that the system has reached a state of equilibrium. The temperature of this equilibrium is the freezing point or melting point. [Local]

Any phase change for a pure substance can produce a similar situation: melting of a solid and freezing of its liquid; evaporation of a liquid and condensation of its vapor; and sublimation of a solid and deposition of its vapor. All can reach an equilibrium condition.