The oldest rocks in the group are in the north west of Scotland, Ireland, and North Wales and are 2.7 billion years old. During the Silurian period, the north-western regions collided with the south-east, which had been part of a separate continental landmass. The topography of the islands is modest in scale by global standards. Ben Nevis rises to an elevation of only 1,345 metres (4,413 ft), and Lough Neagh, which is notably larger than other lakes in the island group, covers 390 square kilometres (151 sq mi). The climate is temperate marine, with mild winters and warm summers. The North Atlantic drift brings significant moisture and raises temperatures 11 °C (20 °F) above the global average for the latitude. This led to a landscape which was long dominated by temperate rainforest, although human activity has since cleared the vast majority of forest cover. The region was re-inhabited after the last glacial period of Quaternary glaciation, by 12,000 BC, when Great Britain was still part of a peninsula of the European continent. Ireland, which became an island by 12,000 BC, was not inhabited until after 8000 BC. Great Britain became an island by 5600 BC.
Hiberni (Ireland), Pictish (northern Britain) and Britons (southern Britain) tribes, all speaking Insular Celtic, inhabited the islands at the beginning of the 1st millennium AD. Much of Brittonic-occupied Britain was conquered by the Roman Empire from AD 43. The first Anglo-Saxons arrived as Roman power waned in the 5th century, and eventually dominated the bulk of what is now England.Viking invasions began in the 9th century, followed by more permanent settlements and political change, particularly in England. The Norman conquest of England in 1066 and the later Angevin partial conquest of Ireland from 1169 led to the imposition of a new Norman ruling elite across much of Britain and parts of Ireland. By the Late Middle Ages, Great Britain was separated into the Kingdoms of England and Kingdom of Scotland, while control in Ireland fluxed between Gaelic kingdoms, Hiberno-Norman lords and the English-dominated Lordship of Ireland, soon restricted only to The Pale. The 1603 Union of the Crowns, Acts of Union 1707 and Acts of Union 1800 attempted to consolidate Britain and Ireland into a single political unit, the United Kingdom of Great Britain and Ireland, with the Isle of Man and the Channel Islands remaining as Crown Dependencies. The expansion of the British Empire and migrations following the Irish Famine and Highland Clearances resulted in the dispersal of some of the islands' population and culture throughout the world, and a rapid depopulation of Ireland in the second half of the 19th century. Most of Ireland seceded from the United Kingdom after the Irish War of Independence and the subsequent Anglo-Irish Treaty (1919–1922), with six counties remaining in the UK as Northern Ireland.
The earliest known references to the islands as a group appeared in the writings of sea-farers from the ancient Greek colony of Massalia. The original records have been lost; however, later writings, e.g. Avienus's Ora maritima, that quoted from the Massaliote Periplus (6th century BC) and from Pytheas's On the Ocean (circa 325–320 BC) have survived. In the 1st century BC, Diodorus Siculus has Prettanikē nēsos, "the British Island", and Prettanoi, "the Britons".Strabo used Βρεττανική (Brettanike), and Marcian of Heraclea, in his Periplus maris exteri, used αἱ Πρεττανικαί νῆσοι (the Prettanic Isles) to refer to the islands. Historians today, though not in absolute agreement, largely agree that these Greek and Latin names were probably drawn from native Celtic-language names for the archipelago. Along these lines, the inhabitants of the islands were called the Πρεττανοί (Priteni or Pretani). The shift from the "P" of Pretannia to the "B" of Britannia by the Romans occurred during the time of Julius Caesar.
The Greco-Egyptian scientist Claudius Ptolemy referred to the larger island as great Britain (μεγάλη Βρεττανία megale Brettania) and to Ireland as little Britain (μικρὰ Βρεττανία mikra Brettania) in his work Almagest (147–148 AD). In his later work, Geography (c. 150 AD), he gave these islands the names Alwion, Iwernia, and Mona (the Isle of Man), suggesting these may have been names of the individual islands not known to him at the time of writing Almagest. The name Albion appears to have fallen out of use sometime after the Roman conquest of Great Britain, after which Britain became the more commonplace name for the island called Great Britain.
The earliest known use of the phrase Brytish Iles in the English language is dated 1577 in a work by John Dee. Today, this name is seen by some as carrying imperialist overtones although it is still commonly used. Other names used to describe the islands include the Anglo-Celtic Isles,Atlantic archipelago, British-Irish Isles,Britain and Ireland, UK and Ireland, and British Isles and Ireland. Owing to political and national associations with the word British, the Government of Ireland does not use the term British Isles and in documents drawn up jointly between the British and Irish governments, the archipelago is referred to simply as "these islands". Nonetheless, British Isles is still the most widely accepted term for the archipelago.