The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.
WRCC Climate and Weather Terms Glossary

Latent Heat - If a change of state occurs from gas to liquid or liquid to solid, internal energy in the form of heat is released. If a change of state occurs from solid to liquid or liquid to gas, heat is required. Different compounds absorb and release different amounts of latent heat. Water releases nearly 600 kilocalories for each kilogram of water condensed.
[Journal of Applied Meteorology; v33n4; 477-489; 1994.]
[A Field Guide to the Atmosphere; V. Schaefer, J. Day; page 75; 1981; Houghton Mifflin Company; Boston.]
[Air, The Nature of Atmosphere and the Climate; Michael Allaby; pages 59,60; 1992; Facts on File; New York.] 
Atmospheric Chemistry Glossary