Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was introduced around 1762 by Joseph Black. It is derived from the Latin latere (to lie hidden). Black used the term in the context of calorimetry when referring to the heat transferred that caused a change of volume while the thermodynamic system was held at constant temperature.

In contrast to latent heat, an energy is called a sensible energy or heat, when it causes processes that do result in a change of the temperature of the system.

Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting or freezing) and latent heat of vaporization (boiling or condensing). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and to gas.