Lipid, LIHP ihd or LY pihd, is one of a large group of oily or fatty substances essential for good health. Lipids, carbohydrates, and proteins are the classes of compounds in all living things. Animal fats and plant oils are lipids. So are animal sex hormones and vitamins A, D, E, and K. Egg yolks, liver, and embryos of grains are rich in lipids.