The Anglo-Saxon Chronicle, under the year 827 (for 829), states that:

… King Egbert subdued the kingdom of the Mercians, and all that was south of the Humber, and he was the eighth king who was Bretwalda.[*] The first was Ælle, king of the South Saxons [r.477?–514?], who had thus much sway; the second was Ceawlin, king of the West Saxons [r.560?–591?]; the third was Æthelberht, king of the people of Kent [r.560?–616]; the fourth was Rædwald, king of the East Angles [r.599?–624?]; the fifth was Edwin, king of the Northumbrians [r.616–633]; the sixth was Oswald, who reigned after him [r.634–642]; the seventh was Oswiu, Oswald’s brother [r.642–670]; the eighth was Egbert, king of the West Saxons [r.802–839].

The anonymous West Saxon author of this Chronicle entry has taken his list of the seven Bretwaldas prior to Egbert from Bede. Bede, however, makes no mention of the word Bretwalda, nor, indeed, any other title:

He [Æthelberht] was the third of the English kings who ruled over all the southern provinces that are divided from the northern by the river Humber and the borders contiguous to it; but the first of all that ascended to the heavenly kingdom. The first who had the like sovereignty was Ælle, king of the South Saxons; the second, Cælin, king of the West-Saxons, who, in their own language, is called Ceawlin; the third, as has been said, was Æthelberht, king of the people of Kent; the fourth was Rædwald, king of the East Angles, who, even in the life-time of Æthelberht, had been acquiring the leadership for his own race. The fifth was Edwin, king of the Northumbrian nation, that is, of those who live in the district to the north of the river Humber; his power was greater; he ruled over all the inhabitants of Britain, both English and Britons, except only the people of Kent; and he reduced also under the dominion of the English, the Mevanian Islands of the Britons [Anglesey and the Isle of Man], lying between Ireland and Britain; the sixth was Oswald, the most Christian king of the Northumbrians, whose kingdom was within the same bounds; the seventh, his brother Oswiu, ruled over a kingdom of like extent for a time, and for the most part subdued and made tributary the nations of the Picts and Scots, who occupy the northern parts of Britain …
Historia Ecclesiastica II, 5

So the defining characteristic of these kings is evidently that they exercised overlordship of all Anglo-Saxon provinces south of the Humber, though Bede then proceeds to make Kent an exception in the case of the Northumbrian kings. Also, Bede states that, at the time of his writing (731), the:

… southern provinces, as far as the boundary formed by the river Humber, with their several kings, are subject to Æthelbald, king of the Mercians.
Historia Ecclesiastica V, 23

Yet Æthelbald (r.716–757) is not included in Bede’s list, nor Æthelbald’s predecessor, Wulfhere (r.658–675), who might be supposed to have fitted the bill. In the period after Bede’s death, the Chronicle doesn’t consider Offa of Mercia (r.757–796) to be suitably qualified to add to the list. Just what constitutes a Bretwalda is, therefore, not easy to state categorically, and is a subject of continuing scholarly debate.

Anglo-Saxon Chronicle adapted from the translation of Benjamin Thorpe
Bede Historia Ecclesiastica Gentis Anglorum adapted from the translation of A.M. Sellar

Actually, the form Bretwalda only appears in Manuscript A, the oldest copy of the Chronicle (and hence is the form usually adopted by modern writers). In fact, the scribe first wrote Bretwala, and then inserted the d between the l and the a above the line, which does not inspire confidence in his transcription of the word. The other manuscripts have variations of the form Brytenwalda.[*] The word element, walda, means ‘ruler’ (from wealdan, ‘to rule, govern’, source of modern English ‘to wield’). Bretwalda is translated as ‘Britain-ruler’, i.e. ‘ruler of Britain’. Brytenwalda could also mean ‘ruler of Britain’, but could possibly have the meaning ‘wide-ruler’, i.e. ‘ruler over a wide area’ (but no claim to the whole of Britain).[*]
In a 12th century Winchester cartulary (British Library Add. MS 15350) there is a faked charter (S427) of King Athelstan (r.924–939) which appears in both Latin and Old English versions. In Latin Athelstan is styled rex et rector totius huius britannie insule (‘king and ruler of all this island of Britain’); in Old English he is ongol saxna cyning 7 brytæn walda eallæs ðyses Iglandæs (‘king of the Anglo-Saxons and Bretwalda of all this island’).
Simon Keynes* explains: “The element bryten certainly could in these cases be the noun meaning ‘Britain’, in which case Brytenwealda would mean precisely ‘ruler of Britain’; the form in [Manuscript] C, with -anwealda, would then give the meaning ‘sole ruler of Britain’. On the other hand, the first element might be the adjective bryten (from the verb breotan, meaning ‘to break, disperse’), often used in poetic compounds to denote the wide or general dispersion of the noun that it qualifies; so that just as bryten-rice means an extensive kingdom, and bryten-grund the wide expanse of the earth, so too does bryten-cyning mean a king whose authority was widely extended.”
* ‘Rædwald the Bretwalda’, in Voyage to the Other World: the Legacy of Sutton Hoo (1992), p.111.
Manuscript B has Bryten walda.
Manuscript C has Bretenanwealda.
Manuscripts D and E have Bryten wealda.
Manuscript F has Bryten weald.
Anglo-Saxon charters are referred to by their number in Sawyer’s catalogue.
Available online: The Electronic Sawyer.