The Roman province (and later diocese) that included most of the later England and all of Wales, and at its greatest extent reached into central Scotland. It was called after the people living there.
By the reign of Septimus Severus, the province was divided into Britannia Superior and Britannia Inferior, south and north of a line from the Wash to the Mersey estuary. Britannia Superior – Upper Britannia – is lower than Lower Britannia, except for the Welsh Mountains, but it was nearer to Rome: a similar principle is operating in this respect as once made the railway lines to London the up lines.
At the end of the 3rd century Britannia became a diocese divided into four provinces. Maxima Caesariensis occupied what is now the southeastern quarter of England; Flavia Caesariensis the English Midlands and the lands either side the Wash; Britannia Prima Wales and the West Country; and Britannia Secunda the lands north of Mersey to Humber.
The name later extended to the whole island, anglicised as Britain, becoming Great Britain (Britannia Major) when mass immigration gave Brittany its name (Britannia Minor). Britannia remains a poetical name for the island, personified on UK coinage by a strapping lady in Roman headgear, originally la Belle Stuart, one of the mistresses of Charles II.