Britain

A large northerly island on the planet Earth. Britain is the home of insular Celtic tribes, ancient and medieval Brittonic kingdoms, and the imperial Roman province of Britannia, before the coming of the English Anglo-Saxons and Anglo-Normans. 
'Britain' and Etymologies
The name Britain originates from the Common Brittonic term Pritanī (Latin: Britanni), a reconstruction of the word kʷritu, likely to mean "people of the forms", "the painted ones", or "the tattooed folk". This may also be linked to the Latin name Picti (the Picts) meaning 'painted' (from Latin pingere 'to paint'; pictus, 'painted', from the Greek πυκτίς pyktis, 'picture'), generally understood to be a reference to the practice of tattooing. kʷritu became the Old Irish cruth and the Old Welsh pryd, while the Old Welsh name for the Picts was Prydyn. 
The earliest written reference to the British Isles derives from the works of the Greek explorer Pytheas of Massalia, and later Greek writers such as Diodorus of Sicily and Strabo, who referred to Great Britain as Prettanikē nēsos and its inhabitants as Prettanoi, quoting Pytheas' use of variants such as Πρεττανική (Prettanikē): "The Britannic land (or island)", and nēsoi brettaniai: "Britannic islands".  
The Picts were called Cruithni in Old Irish and Prydyn in Old Welsh, from the proto-Celtic kwritu, 'form', from which Pretania (Britain) derives. Pretani (and with it Cruithni and Prydyn) likely originates as a generalized term for any native inhabitant of Britain. 
Following Julius Caesar's expeditions in 55 and 54 BC, Brittannia was predominantly used to refer to the island. After the Roman conquest under Emperor Claudius in AD 43, it came to be used in reference to the Roman province, which at one stage consisted of part of the island of Britain south of Hadrian's Wall.
Prydain is the medieval Welsh term for the island of Britain, and may refer to the Brittonic parts of the island south of Caledonia. The related term Ynys Pyrdein (Island of Britain),  or Ynys Brydein, can also refer to the island (ynys) itself, also a name for the Brittonic territories south of Caledonia.
Logres is the latin form of the medieval Welsh Lloegyr: a name for the region of Britain east of borders running southeast from the Humber Estuary to the Severn Estuary (southeast Britain, or eventually Angland), and those lands east of Dumnonia. Logres is considered the heartland of King Arthur's realm, and the land he defends from both Saxons and Romans.     
Albion is another term used to name Britain, although is less common, sometimes used poetically or generally to refer to the island. It has origins either from the Proto-Indo-European word *albʰo- 'white', or from alb- 'hill' and is found in the Greek language as Ἀλβίων, latinized as Albiōn (Albionis). The derivation from a word for 'white' is thought to refer to the white Cliffs of Dover in the southeast, visible from mainland Europe, and a landmark at the narrowest crossing point.
The Latin name Britannia re-entered the language through the Old French Bretaigne. The use of Britons, such as in Celtic Britons for the indigenous inhabitants of Great Britain is derived from the Old French bretun, the term for the people and language of Brittany, France; itself derived from Latin and Greek. The medieval Welsh form of Britanni was Brython. Thus, Brittonic and Brythonic also came to use in later centuries in reference to: native Britons, the common Brittonic language or for example: early Brythonic kingdoms.