maleic acid

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

• noun A colorless crystalline acid, the cis-isomer of C₄H₄O₄, used in textile processing and as an oil and fat preservative.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chem.) An unsaturated dicarboxylic acid (HO.CO.CH=CH.CO.OH) of cis conformation, having various uses in the manufacture of artificial resins, the dyeing and finishing of wool, silk, and cotton, and also combined with certain drugs to make salts more soluble than the basic forms. The corresponding isomer having trans conformation, is identical in chemical formula, is called fumaric acid.

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

• noun organic chemistry an unsaturated organic dicarboxylic acid, HOOCCH=CHCOOH, the geometric isomer of fumaric acid; it is found in unripe fruit, and is used industrially in the production of polyester resins

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

• noun a colorless crystalline compound found in unripe fruit (such as apples or tomatoes or cherries) and used mainly to make polyester resins