We call the change of state temperature of a substance the temperature at which, although the body absorbs or emits heat, the variation in energy is totally used in changing the physical state of the body. While this transformation lasts the temperature of the body does not vary.

In general, bodies have a melting point and a boiling point, although we must remember that these temperatures depend on the outside pressure and the purity of the substance.

We call the energy necessary for the unit mass of a body, which is already at the appropriate temperature to change its state, latent heat (of fusion or boiling). 

A well-known example is distilled water
Melting point at 1 atmosphere of pressure: 0ºC
Boiling point at 1 atmosphere of pressure: 100ºC
Latent heat of fusion: 80 cal/g = 334.400 j/kg
Latent heat of boiling: 540 cal/g = 2.257.200 j/kg