Great Britain and United Kingdom, Explained
The names Great Britain and United Kingdom are often used interchangeably, but are not actually synonymous. Here’s a visual to help you remember.
The British Isles are a group of islands off the northwestern coast of Europe. The largest of these islands are Britain and Ireland. (Smaller ones include the Isle of Wight.)
In the Middle Ages, the name Britain was also applied to a small part of France now known as Brittany. As a result, Great Britain came into use to refer specifically to the island. However, that name had no official significance until 1707, when the island’s rival kingdoms of England and Scotland were united as the Kingdom of Great Britain.
Ireland, meanwhile, had effectively been an English colony since the 12th century, and after the emergence of Great Britain, it remained under the influence of the British crown.
In 1801 it formally joined with Great Britain as a single political entity, which became known as the United Kingdom of Great Britain and Ireland—or the United Kingdom for short.
However, the union lasted only until 1922, when Ireland (with the exception of six counties in the north) seceded. Ireland soon became a sovereign republic, and its former partner took on the official name of the United Kingdom of Great Britain and Northern Ireland.
Source: https://www.britannica.com/story/whats-the-difference-between-great-britain-and-the-united-kingdom