zone melting or zone refiningn. A purification technique for crystalline substances in which a heating system passes slowly over a bar of the material to be refined, creating a molten region that carries impurities with it across the bar.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.