Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe. With an area of 209,331 km² (80,823 sq mi), it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It is dominated by a maritime climate with narrow temperature differences between seasons. The island of Ireland, with an area 40 per cent that of Great Britain, is to the west—these islands, along with over 1,000 smaller surrounding islands and named substantial rocks, form the British Isles archipelago.

Connected to mainland Europe until 9,000 years ago by a landbridge now known as Doggerland, Great Britain has been inhabited by modern humans for around 30,000 years. In 2011, it had a population of about 61 million, making it the world's third-most-populous island after Java in Indonesia and Honshu in Japan.

The term "Great Britain" can also refer to the political territory of England, Scotland and Wales, which includes their offshore islands. This territory and Northern Ireland constitute the United Kingdom. The single Kingdom of Great Britain resulted from the 1707 Acts of Union between the kingdoms of England (which at the time incorporated Wales) and Scotland.