The area of modern-day Saudi Arabia formerly consisted of four distinct regions: Hejaz, Najd and parts of Eastern Arabia (Al-Ahsa) and Southern Arabia ('Asir). The Kingdom of Saudi Arabia was founded in 1932 by Ibn Saud. He united the four regions into a single state through a series of conquests beginning in 1902 with the capture of Riyadh, the ancestral home of his family, the House of Saud. Saudi Arabia has since been an absolute monarchy, effectively a hereditary dictatorship governed along Islamic lines. The ultraconservative Wahhabi religious movement within Sunni Islam has been called "the predominant feature of Saudi culture", with its global spread largely financed by the oil and gas trade. Saudi Arabia is sometimes called "the Land of the Two Holy Mosques" in reference to Al-Masjid al-Haram (in Mecca) and Al-Masjid an-Nabawi (in Medina), the two holiest places in Islam. The state has a total population of 28.7 million, of which 20 million are Saudi nationals and 8 million are foreigners. The state's official language is Arabic.
Petroleum was discovered on 3 March 1938 and followed up by several other finds in the Eastern Province. Saudi Arabia has since become the world's largest oil producer and exporter, controlling the world's second largest oil reserves and the sixth largest gas reserves. The kingdom is categorized as a World Bank high-income economy with a high Human Development Index and is the only Arab country to be part of the G-20 major economies. However, the economy of Saudi Arabia is the least diversified in the Gulf Cooperation Council, lacking any significant service or production sector (apart from the extraction of resources). The state has attracted criticism for its treatment of women and use of capital punishment. Saudi Arabia is a monarchical autocracy, has the fourth highest military expenditure in the world and SIPRI found that Saudi Arabia was the world's second largest arms importer in 2010–2014. Saudi Arabia is considered a regional and middle power. In addition to the GCC, it is an active member of the Organisation of Islamic Cooperation and OPEC.
Following the unification of the Hejaz and Nejd kingdoms, the new state was named al-Mamlakah al-ʻArabīyah as-Suʻūdīyah (a transliteration of المملكة العربية السعودية in Arabic) by royal decree on 23 September 1932 by its founder, Abdulaziz Al Saud (Ibn Saud). Although this is normally translated as "the Kingdom of Saudi Arabia" in English, it literally means "the Saudi Arab kingdom", or "the Arab Saudi Kingdom".
The word "Saudi" is derived from the element as-Suʻūdīyah in the Arabic name of the country, which is a type of adjective known as a nisba, formed from the dynastic name of the Saudi royal family, the Al Saud (آل سعود). Its inclusion expresses the view that the country is the personal possession of the royal family. Al Saud is an Arabic name formed by adding the word Al, meaning "family of" or "House of", to the personal name of an ancestor. In the case of the Al Saud, this is the father of the dynasty's 18th-century founder, Muhammad bin Saud.
There is evidence that human habitation in the Arabian Peninsula dates back to about 125,000 years ago. It is now believed that the first modern humans to spread east across Asia left Africa about 75,000 years ago across the Bab el Mandib connecting Horn of Africa and Arabia.