What is "maleic acid"

maleic acid \maleic acid\ n. (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.
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n. (context organic compound English) 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

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

Maleic acid or cis-butenedioic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HOCCHCHCOH. Maleic acid is the cis-isomer of butenedioic acid, whereas fumaric acid is the trans-isomer. It is mainly used as a precursor to fumaric acid, and relative to its parent maleic anhydride, maleic acid has few applications.