The Fusion is the change in enthalpy for the conversion of 1 mole or 1 gram of a solid to a liquid, at constant pressure and temperature and is usually denoted as ΔHfus.

A physical property

Enthalpy of fusion/vaporization is the amount of energy added to a system to melt of boil a substance or the amount of energy removed from a system to condense or freeze a substance.

heat of fusion

Enthalpy is a state function, and to a first approximation does not depend on temperature. So the change in enthalpy to go from solid to a gas directly (sublimation) at some temperature is equal to the sum of the enthalpies associated with going from a solid to a liquid (fusion) and going from a liquid to a gas (vaporization) at other temperatures.