The word goes back to early Celtic days when only major valleys were so designated in one form or another - valleys that were used as major trade routes. The most glaring example of this is the route used by the expanding Celts to get from the area of the Black Sea, to deep into Central Europe. The route they used still bears the name today - Danube, a corruption of Danu or Denu, still used by some of the nations where the great river passes through.