ma·le·ic acid (m?-l???k)n. A colorless crystalline acid, the cis-isomer of C4H4O4, used in textile processing and as an oil and fat preservative.[From French acide maléique, alteration of acide malique, malic acid; see malic acid.]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.maleic acid (m??le??k) n (Elements & Compounds) a colourless soluble crystalline substance used to synthesize other compounds. Formula: HOOCCH:CHCOOH. Systematic name: cis-butanedioic acid [C19: from French maléique, altered form of malique; see malic acid]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ma?le?ic ac?id (m??li ?k) n. a colorless, crystalline, water-soluble solid, C4H4O4, used to make synthetic resins and to dye textiles. [1870?75;