Decreased functioning in muscle tissue or organ because lack of exercise e.g. astronauts have to exercise daily to keep their muscle from atrophy, because of zero gravity muscle does not get any resistance and don’t grow anymore. Or when you exercise a lot and then stop exercising your muscle will start to atrophy. Body stops wasting energy on muscles you don’t use! Organs also do atrophy for example stomach atrophy gastritis. If you don’t have good diet and don’t eat in required times or miss dinner, your stomach will start to atrophy.