Britain (or more accurately, Great Britain) is the name of the largest of the British Isles, which lie off the northwest coast of continental Europe. The name is probably Celtic and derives from a word meaning 'white'; this is usually assumed to be a reference to the famous white cliffs of Dover, which any new arrival to the country by sea can hardly miss. The first mention of the island was by the Greek navigator Pytheas, who explored the island's coastline, c. 325 BCE.
During the early Neolithic Age (c. 4400 BCE – c. 3300 BCE), many long barrows were constructed on the island, many of which can still be seen today. In the late Neolithic (c. 2900 BCE – c. 2200 BCE), large stone circles called henges appeared, the most famous of which is Stonehenge.
Before Roman occupation the island was inhabited by a diverse number of tribes that are generally believed to be of Celtic origin, collectively known as Britons. The Romans knew the island as Britannia.
It enters recorded history in the military reports of Julius Caesar, who crossed to the island from Gaul (France) in both 55 and 54 BCE. The Romans invaded the island in 43 CE, on the orders of emperor Claudius, who crossed over to oversee the entry of his general, Aulus Plautius, into Camulodunum (Colchester), the capital of the most warlike tribe, the Catuvellauni. Plautius invaded with four legions and auxiliary troops, an army amounting to some 40,000.
Due to the survival of the Agricola, a biography of his father-in-law written by the historian Tacitus (c. 105 CE), we know much about the first four decades of Roman occupation, but literary evidence is scarce thereafter; fortunately, there is plentiful if occasionally mystifying archaeological evidence. Subsequent Roman emperors made forays into Scotland, although northern Britain was never conquered; they left behind the great fortifications, Hadrian's Wall (c. 120 CE) and the Antonine Wall (142 -155 CE), much of which can still be visited today. Britain was always heavily fortified and was a base from which Roman governors occasionally made attempts to seize power in the Empire (Clodius Albinus in 196 CE, Constantine in 306 CE).
At the end of the 4th century CE, the Roman presence in Britain was threatened by "barbarian" forces. The Picts (from present-day Scotland) and the Scoti (from Ireland) were raiding the coast, while the Saxons and the Angles from northern Germany were invading southern and eastern Britain. By 410 CE the Roman army had withdrawn. After struggles with the Britons, the Angles and the Saxons emerged as victors and established themselves as rulers in much of Britain during the Dark Ages (c. 450 - c. 800 CE).