1) The amount of heat required to melt one gram of solid at its melting point with no change in temperature. Usually expressed in J/g. The molar heat of fusion is the amount of heat required to melt one mole of a solid at its melting point with no change in temperature and is usually expressed in kJ/mol.
2) The heat required to convert a solid into a liquid with no temperature change. This is also called the latent heat of fusion or melting.