Ester, an organic compound that is formed by the reaction of an alcohol with an acid. This reaction is called esterification. Animal fats and vegetable oils are esters of fatty acids and glycerol (glycerin), a type of alcohol. The flavor and fragrance of bananas, pears, apricots, and certain other fruits is due largely to specific esters they contain. Important synthetic esters include ethyl acetate, used as a solvent, and polyester resins, used in making textile fibers and many other products.

Esters undergo hydrolysis (chemical decomposition in the presence of water) to form acids and alcohols. When animal fats are heated in a solution containing caustic soda (sodium hydroxide), they are converted into soap and glycerol. This reaction is called saponification.