The quantity of heat absorbed or released per unit mass when a substance changes its physical phase at constant temperature and pressure. The specific latent heat of fusion (specific enthalpy change on fusion) of a body is the heat required to convert one kilogram of the solid at its melting point into liquid at the same temperature. The specific latent heat of vaporization (specific enthalpy change on vaporization) of a liquid is the heat required to convert one kilogram of the liquid at its boiling point into vapour at the same temperature. The SI units are J kg⁻¹.