The Collaborative International Dictionary of English v.0.48:

Bretwalda \Bret"wal*da\, n. [AS. Bretwalda, br?ten walda, a
   powerful ruler.] (Eng. Hist.)
   The official title applied to that one of the Anglo-Saxon
   chieftains who was chosen by the other chiefs to lead them in
   their warfare against the British tribes. --Brande & C.
   [1913 Webster]