Final answer:
The island bordered by the North Sea, North Atlantic, Irish Sea, Celtic Sea, and English Channel is Great Britain, which is part of the United Kingdom alongside Scotland, Wales, and Northern Ireland.
Step-by-step explanation:
The island that is bordered by the North Sea to the east, the North Atlantic on the north, the Irish Sea, and the Celtic Sea on the west and southwest, and the English Channel on the south is Great Britain.
The British Isles, which include Great Britain, are an archipelago off the northwestern coast of continental Europe. While these islands include both Great Britain and the Republic of Ireland, it is Great Britain that fits the description given in the question.
Great Britain itself is part of a larger country known as the United Kingdom (UK), which also includes Scotland, Wales, and Northern Ireland.