heat of vaporization

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

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

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

• noun physics The amount of heat needed to change a unit mass of material from a liquid to a gas at its boiling point

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

• noun heat absorbed by a unit mass of a material at its boiling point in order to convert the material into a gas at the same temperature