The instrument a doctor or nurse puts in their ears, with a small round metal disc on the end of a rubber tube, so they can hear the sounds inside your body — your heart beating, your lungs when you breathe in, sometimes your stomach. The doctor presses the cold disc on your chest or your back and says "breathe in, breathe out." It does not see anything; it only listens. That is worth saying because students sometimes think it is a scanner of some kind. It is closer to putting your ear on someone's chest — which is exactly what doctors did before this was invented. It is a very ordinary, everyday word in a hospital, not a technical one, and it is the thing everyone pictures when they picture a doctor at all: hanging around the neck in every hospital drama, on the sign outside a clinic. Because of that it has become a kind of symbol — if you see a stethoscope in a picture, you know the picture is about medicine. You almost never meet the word outside of doctors, animals doctors, and pictures of them. It is a noun. There is a verb 'to stethoscope', but honestly you can go your whole life without hearing anyone use it, and you should not.