Greek and Roman periods
The first writer to use a form of the name was the Greek explorer and geographer Pytheas in the 4th century BC. Pytheas referred to Prettanike or Brettaniai, a group of islands off the coast of North-Western Europe. In the 1st century BC, Diodorus Siculus referred to Pretannia, a rendering of the indigenous name for the Pretani people whom the Greeks believed to inhabit the British Isles. Following the Greek usage, the Romans referred to the Insulae Britannicae in the plural, consisting of Albion (Great Britain), Hibernia (Ireland), Thule (possibly Iceland or Orkney) and many smaller islands. Over time, Albion specifically came to be known as Britannia, and the name for the group was subsequently dropped.
Although emperor Claudius is commonly attributed with the creation and unification of the province of Britannia in 43 AD, Julius Caesar had already established Roman authority over the Southern and Eastern Britain dynasties during his two expeditions to the island in 55 and 54 BC. Just as Caesar himself had been an obside in Bithynia as a youth, he also had taken the King's sons as obsides or hostages, back to Rome, partially to be educated.
The Roman conquest of the island began in AD 43, leading to the establishment of the Roman province known in Latin as Britannia. The Romans never successfully conquered the whole island, building Hadrian's Wall as a boundary with Caledonia, which covered roughly the territory of modern Scotland, although the whole of the boundary marked by Hadrian's Wall lies within modern-day Northern England. A southern part of what is now Scotland was occupied by the Romans for about 20 years in the mid-2nd century AD, keeping in place the Picts to the north of the Antonine Wall. People living in the Roman province of Britannia were called Britanni, or Britons. Ireland, inhabited by the Scoti, was never invaded and was called Hibernia. Thule, an island "six days' sail north of Britain, and [...] near the frozen sea", possibly Iceland, was also never invaded by the Romans.
coin from the reign of Antoninus Pius struck in 154 AD showing Britannia on the reverse
The Emperor Claudius paid a visit while Britain was being conquered and was honoured with the agnomen Britannicus as if he were the conqueror; a frieze discovered at Aphrodisias in 1980 shows a bare breasted and helmeted female warrior labelled BRITANNIA, writhing in agony under the heel of the emperor. She appeared on coins issued under Hadrian, as a more regal-looking female figure. Britannia was soon personified as a goddess, looking fairly similar to the goddess Minerva. Early portraits of the goddess depict Britannia as a beautiful young woman, wearing a Corinthian helmet, and wrapped in a white garment with her right breast exposed. She is usually shown seated on a rock, holding a trident, and with a spiked shield propped beside her. Sometimes she holds a standard and leans on the shield. On another range of coinage, she is seated on a globe above waves: Britain at the edge of the (known) world. Similar coin types were also issued under Antoninus Pius.