Great Britain, together with the province of Northern Ireland and the many Overseas Territories form the United Kingdom. Therefore, Great Britain is part, but only part, of the United Kingdom.
People often wonder why the island is called Great Britain rather than just Britain. The reason for this is that there are two Britains: the island of Britain in the British Isles; and the land of Britain in France. In French these are known as Grande Bretagne and Bretagne; in English, Great Britain and Brittany.
The word Great in this context has its old meaning of big as in she was great with child or Greater London. Likewise, the ending -y on the end of Brittany has the meaning Little as in doggy meaning small dog or Jimmy meaning little Jim.
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