ester - chemical compound formed by the reaction of an organic or inorganic acid with an alcohol or with another organic compound containing the hydroxyl (-OH) radical. The reaction involves replacement of the hydrogen of the acid with a hydrocarbon group. The name of an ester indicates its derivation; e.g., the ester resulting from the reaction of ethyl alcohol and acetic acid is called ethyl acetate. Esters have important uses in the formulation of some petroleum additives and synthetic lubricants. See dibasic acid ester, petroleum, phosphate ester.