When a solid body is heated, its temperature rises at a rate determined by the rate at which heat is being supplied and the heat capacity of the body. However, at certain temperatures, the body starts to change its phase, and as phase change proceeds the temperature remains constant even though heat is being continually supplied. Illustrated in figure 1 from point A to B and from D to E. The heat supplied during this period of constant temperature is called heat of melting.

Figure 1: heat supplied vs temperature