The bit of Europe seen from Calais, looking west across the English Channel. When the land bridge connecting it to the rest of Europe collapsed about 9000 years ago, it left Europe stranded. The earliest Celt settlers were a tribe called the Brythons.
Geographically, the largest island of the British Isles.
Politically, the collection of two countries and a principality. They being, England, Scotland and Wales and including smaller islands. Northern Ireland joins in the United Kingdom but not Great Britain.