Fatty acids: Molecules that are long chains of lipid-carboxylic acid found in fats and oils and in cell membranes as a component of phospholipids and glycolipids. (Carboxylic acid is an organic acid containing the functional group-COOH.). Fatty acids come from animal and vegetable fats and oils.Fatty acids play roles outside the body; they are used as lubricants, in cooking and food engineering, and in the production of soaps, detergents, and cosmetics.Carboxylic acid is an organic acid containing the functional group-COOH.). Fatty acids come from animal and vegetable fats and oils. Fatty acids play roles outside the body; they are used as lubricants, in cooking and food engineering, and in the production of soaps, detergents, and cosmetics.