Ester is an organic compound derived from an acid in which the hydroxyl group is replaced by an alkoxy group. Oleate is an ester of oleic acid, a monounsaturated fatty acid found in many animal and vegetable fats. Sterol is a type of lipid found in the cell membranes of all animals and is a precursor to steroid hormones. Lactate is an ester of lactic acid, a three-carbon molecule produced during anaerobic respiration. All of these compounds are related to the term ester, which is an organic compound derived from an acid in which the hydroxyl group is replaced by an alkoxy group.