Bretwalda

The term “bretwalda” (also “brytenwalda” in various versions) first appeared in the Anglo-Saxon Chronicle’s report on AD 829, referring to king Egbert as “ruler of Britain”. “Brytenwalda” (Britain-wide) was later used in a charter issued by Athelstan in 934 where the English version had “brytenwalda” while the Latin copy of the text had “king and ruler of the whole island of Britain”.

Whatever... “bretwalda” has gained currency with historians as the word to use when describing certain powerful rulers of that era, especially the seven who, according to Bede, had “imperium” over the island at various times from the late 5th to the 7th centuries. Almost all of these seven - described as the "heptarchy" in much later attempts to make simple sense of the political complications of this period with a simple learned sounding phrase - are recorded as having some sort of dominance at some stage or another.

The term doesn’t mean direct rule but implies “overlordship” and the right to demand tribute

References:

"The Anglo-Saxon Chronicle" trans. Anne Savage.

“Oxford Companion to British History”