Mercians: the men of Mercia, or land of the Middle English.
supremacy: Northumbria, Mercia, Wessex, were separate kingdoms which were successively, in the order in which they are given, strong enough to overawe or exercise supremacy over the others. The king of Wessex eventually became king of England.
Witan, or in its fuller form wit-an-a-ge-mote, the 'meeting of wise men,' was the national council which afterwards grew up into our modern parliament.
[9. THE SECOND SAXON SETTLEMENT.]
Pagan: heathen, not yet converted to Christianity.
King Alfred, called the Great, was king of England from 871-901 A.D.
Alderman in early England meant the ruler of a large district, such as a shire or kingdom. When Mercia became subject to Wessex it was ruled by an alderman.
Benfleet: a place in Essex, on the north bank of the Thames, not far from Southend.
Brunanburgh was the scene of a defeat of the Danes by Athelstan in 937 A.D.; the place cannot now be identified.