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.