Fusion is the transition of a substance from the solid to the liquid aggregate state. For pure substances, this process takes place at constant pressure and always at a certain temperature, called melting point. During the transition, the temperature remains constant and all the heat supplied is invested in the change of state. For example, a water/ice mixture always has a temperature of 0°C. The amount of energy required for this is called melting heat. The liquid result is often called melt.