Reunification is the act of joining parts together again, especially the parts of a country that was split. The famous news example is German reunification: East Germany and West Germany became one country again in 1990. People also talk about the reunification of Korea, which has not happened. The word feels formal. You will see it in history books and news, not much at the dinner table. Though actually, a company or church that split and later joined again can be given this word too, but “split country becomes one again” is the meaning students meet first. It names the event or the process, not a family party.