Bretwalda is an Old English word. The first record comes from the late 9th-century Anglo-Saxon Chronicle. It is given to some of the rulers of Anglo-Saxon kingdoms from the 5th century onwards who had achieved overlordship of some or all of the other Anglo-Saxon kingdoms. It is unclear whether the word dates back to the 5th century and was used by the kings themselves or whether it is a later, 9th-century, invention. The term bretwalda also appears in a 10th-century charter of Æthelstan. The literal meaning of the word is disputed and may translate to either 'wide-ruler' or 'Britain-ruler'.

Mercia was one of the three main Anglic kingdoms founded after Sub-Roman Britain was settled by Anglo-Saxons in an era called the Heptarchy. It was centred on the River Trent and its tributaries, in a region now known as the Midlands of England.