Emaciated means extremely thin, and the reason matters: the body has lost its flesh from lack of food or from illness, not from choosing to be slim. You can often see the shape of bones. Doctors, aid workers, and news writers use this word because it is precise and serious. You would not call a healthy thin person emaciated, and you would not use it as a joke about a missed lunch. Though actually, people do use it for a pet or a person who has become terribly thin after sickness, even if food is now available again. It describes bodies. It is not a fashion word, and it is stronger than thin or skinny.