Any group of organic compounds, including the fat?s, oils, waxes, sterols, and triglycerides, that are insoluble in water but soluble in nonpolar organic solvents, are oily to the touch, and together with carbohydrates and proteins constitute the principal structural material of living cells.

Lipids are molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids are energy storage, as structural components of cell membranes, and as signaling molecules.

Biological lipids usually fall into one of two biochemical sub-units: ketoacyl and isoprene groups. Ketoacyls may be divided into six categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids and polyketides. Isoprenes can be divided into two categories: sterol lipids and prenol lipids.

Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids also include molecules like fatty acids and their derivatives such as tri-, di-, and monoglycerides and phospholipids, as well as other sterol-containing metabolites such as cholesterol.