Britain

Around 325 BCE, the Greek explorer Pytheas of Massalia sailed north from the Mediterranean and became the first literate observer to describe the islands he found beyond Gaul. He called them Prettanikai nesoi, the Pretanic islands. The name likely came from a Brittonic tribal or geographic term that Pytheas transcribed as he heard it. His account survived only in fragments, quoted by later writers who trusted and disputed him in equal measure.

The Romans Latinized the name as Britannia when Julius Caesar made his first incursion in 55 BCE. Caesar's two expeditions produced no permanent occupation, but they produced the name. By the time Claudius's legions invaded in 43 CE and established a province, Britannia had become the official Roman designation, appearing on coins, milestones, and administrative documents for nearly four centuries.

After Roman withdrawal in the early 5th century, the name persisted in Latin ecclesiastical writing. The Venerable Bede used Britannia in his Ecclesiastical History, completed in 731 CE, describing it as an island 800 miles long and 200 miles wide. Medieval Welsh literature preserved a related form, Prydein, connecting the name back toward the pre-Roman Celtic world from which Pytheas had first recorded it.

Britain as an English word came through Old French Bretaigne and settled into common use by the 14th century, referring to the ancient Roman province. The political entity Great Britain dates to 1707, when the Acts of Union joined the Kingdom of England and the Kingdom of Scotland. The Great distinguished it from Brittany in France, settled by migrants from the island centuries earlier. A name that Pytheas heard on a shore became the name of a state.