Maleic acid is a dicarboxylic acid with the chemical formula C4H4O4. It's a white, crystalline solid that is soluble in water. It's used as a starting material for the production of various chemicals, including unsaturated polyester resins, alkyd resins, and maleic anhydride.
It's used as a food additive to enhance the flavor of certain foods and in the production of some pharmaceuticals.