as you noted, is the old name of the DR of Congo. It comes from the word "zaire" which is the Portugese variant of the Bantu nzari = river. The first European explorers thought the river Congo was named Zaire. It was only later that they learned that nzari was the actual word for "river" and the name of that particular nzari was Congo.
[Also DR = democratic republic, though my limited experience shows that when a country puts Democratic in its name, one has to be suspicious]