What the Romans called Britain, borrowing the name, as with much else, from the Greeks. In the main the Romans wrote it as Britannia following Caesar's example, but subsequent writers sometimes preferred Brittania and the use of the forms Brittannia and Britania is not unknown. (1)
There is no certainty regarding the derivation of the name, whether it was an invention of the Greeks or derived from the name given to the island by the natives.(2)
It was in the first century, under the emperor Claudius that the island of Britain became the Roman province of Britannia, following the successfull invasion of 43 AD. (3) By the end of the second century, the island was divided into two provinces named Britannia Superior and Britannia Inferior. A century later the Diocletian reforms created four provinces,
- Britannia Prima,
- Britannia Secunda,
- Flavia Caesiensis and
- Maxima Caesiensis
which combined to form the diocese of the Britains, part of the
Gallic prefecture. This political entity of the British diocese was
known in its plural form as
Britanniae, or "the Britains", referring to the multiplicity of provinces.
Technically speaking therefore, Britannia should refer to the geographical island of Britain, whereas Britanniae should be used to refer to the political entity of Roman Britain. However the Romans were apparently, inconsistent in their usage, and often used Britannia to mean either or both.
Notes
(1) Which makes me, at least feel a whole lot better. I've had a devil of a job trying to remember how many 't's and 'n's are in the damn word. It also explains why its De Excidio Britanniae but Historia Brittonum.
(2) Some have argued that the name is a derivation from "pict". (Based on an unsupported idea of the Picts as the "original" inhabitants.)
But it takes some leap of the imagination to think that "pict" could ever be converted into "brit" or event "prit"
(3) Although the Caledonian highlands always remained outside the sphere of direct Roman control, and remained under the dominion of the Picts.However precisely where we can place the boundary between Roman and Pictish territory is a matter of debate.
And even a fifth province Valentia, although the only source for this is, I believe the Notitia dignitatum, so no one is quite sure what part of Britain it was.