Matter exists in 3 states namely: Solid, Liquid, Gas. When energy is absorbed as heat by a solid or liquid, the temperature of the substance does not necessarily rise.

Instead, the substance may change from one phase to another. That is, a solid changes to liquid and liquid changes to gas. In general, the latent heat of vaporization is the absorption or release of heat during the change of state. Usually the change of state occurs from Solid to liquid (vice versa) or liquid to gas (vice versa). In both the cases one can define Latent heat in two different ways.

The amount of energy required to change a unit mass of any substance from solid to liquid (and vice versa) is called the latent heat of fusion for the substance. The amount of energy required to change any substance from liquid to gas (and vice versa) is called the latent heat of vaporization for the substance.

The boiling water absorbs 540 calories (2255 joules) of heat energy to vaporize the one gram. As long as the energy in the form of heat is added to the system, the temperature of the boiling water will be still at 100°C until no water is left to be steam.