If you have access, take a look at W. R. Jones' article "The Image of the Barbarian in Medieval Europe." It gives a good sense of the terminology shifted over time. To very briefly summarize, "barbarian" originally described cultural/societal inferiority; with the rise of Christianity, especially among formerly "barbarian" peoples, the term came to have a more religious meaning: pagan, heathen, etc. With the Renaissance, the classical definition came back into popularity, along with an economic dimension.