Upper Saxony
It was particularly to distinguish the lands from 'Lower Saxony', a concept which arose later in popular usage (though never enforced) as a term for the original Saxon lands in north and west Germany (where Low German dialects had spread), in what is now the state of Lower Saxony, as well as the adjacent Westphalian region, Holstein and the western part of today's Saxony-Anhalt north of the Harz range (Eastphalia).
Upper Saxony
The name derives from when, after the fall of Duke Henry the Lion in 1180, the medieval Duchy of Saxony dissolved and the Duchy of Saxe-Wittenberg lands passed to the House of Ascania and later to the Wettins in the Margraviate of Meissen. These dynasties subdued the areas east of the Saale river inhabited by Polabian Slavs, and took the tribal name "Sachsen" (Saxons) upstream the Elbe with them.