heat of fusion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The amount of heat required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The amount of heat absorbed by unit mass of a material, at its melting point, that causes a phase transition from solid to liquid

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun heat absorbed by a unit mass of a solid at its melting point in order to convert the solid into a liquid at the same temperature