Where and when did Arabia get its name change to Saudi?

Arabia is a piece of land. Saudi Arabia is a country. Since Saudi Arabia occupies over 80% of Arabia, many people use the terms interchangeably even though they are not so. Arabia is still called Arabia, even though its geopolitical divisions have changed often in the last 500 years.

As for Saudi Arabia, the story is more interesting. In 1920, the British recognized the Kingdom of Nejd in central Arabia, which was ruled by the Saud familiy. The Saud proceeded to attack and conquer the neighboring Arabian Kingdom of Hejaz in 1921-1922. The Union of Nejd and Hejaz was renamed as the Kingdom of Saudi Arabia, to recognize that it was all of Arabia under the Saud family in 1924.