Esters are organic compounds formed by the reaction of an acid, usually a carboxylic acid, with an alcohol. They are characterized by the presence of an oxygen-carbon double bond, known as an ester linkage. Esters have a wide variety of uses in industry and are found in many natural products, such as essential oils and fats. They also play an important role in the flavour and fragrance industries, as many esters have pleasant odours and tastes.