The Greco-Egyptian scientist 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).

Geoffrey of Monmouth in his pseudohistorical Historia Regum Britanniae (c.1136) refers to the island of Great Britain as Britannia major ("Greater Britain"), to distinguish it from Britannia minor ("Lesser Britain"), the continental region which approximates to modern Brittany and had been settled in the fifth and sixth centuries by Celtic Briton migrants from Great Britain

The term Great Britain was first used officially in 1474, in the instrument drawing up the proposal for a marriage between Cecily, daughter of Edward IV of England, and James, son of James III of Scotland, which described it as "this Nobill Isle, callit Gret Britanee". The Scottish philosopher and historian, John Major (Mair), published his 'History of Great Britain, both England and Scotland' (Historia majoris Britanniae, tam Angliae quam Scotiae) in 1521. While promoting a possible royal match in 1548, Lord Protector Somerset said that the English and Scots were, "like as twoo brethren of one Islande of great Britaynes again." In 1604, James VI and I styled himself "King of Great Brittaine, France and Ireland".