Terra Nullius is a term the Catholic Church used to describe the Americas in the early 1500s. It means “land inhabited by no one.” The Church had ruled that since we were heathens, we had no souls; and since we had no souls, we were no one. Thus, the land was “empty.”
Terra Nullius goes hand in hand with the Doctrine of Discovery, wherein the Church very quickly decided that all “empty” land belongs to the Church by default.
The Doctrine of Discovery is still on the books today.