A lipid is a fatty substance found in living things. It is the science word that covers a whole group: the fat in your body, cooking oil, wax, cholesterol, and the thin fatty wall around every one of your cells. You will meet it in biology class, in a health report, or on a doctor's paper — 'blood lipids' means the fats floating in your blood, and a 'lipid profile' is the blood test that measures them. Almost always plural: lipids. It is not a word for the dinner table. Nobody says 'this pork has a lot of lipids'; they say fat. The reason science needs a separate word is that 'fat' in ordinary English means the greasy stuff you can see and touch, while lipids include things that do not look fatty at all, like the wax on a leaf. What ties them together is that they do not mix with water — that is really the definition. Oil floating on top of soup is a lipid refusing to join the water, and that same refusing is exactly why your cell walls work: they keep the water outside out and the water inside in.