Iran (/ɪˈrɑːn/ or /aɪˈræn/; Persian: ایران [ʔiːˈɾɑn] ( )), also known as Persia (/ˈpɜrʒə/ or /ˈpɜrʃə/), officially the Islamic Republic of Iran, is a country in Western Asia. It is bordered on the northwest by Armenia, Azerbaijan; with Kazakhstan and Russia across the Caspian Sea; on the northeast by Turkmenistan; on the east by Afghanistan and Pakistan; on the south by the Persian Gulf and the Gulf of Oman; and on the west by Turkey and Iraq. Comprising a land area of 1,648,195 km2 (636,372 sq mi), it is the second-largest nation in the Middle East and the 18th-largest in the world; with 78.4 million inhabitants, Iran is the world's 17th most populous nation. It is the only country that has both a Caspian Sea and Indian Ocean coastline. Iran has been of geostrategic importance because of its central location in Eurasia and Western Asia and the Strait of Hormuz.
Iran is home to one of the world's oldest civilizations, beginning with the formation of the Proto-Elamite and Elamite kingdom in 3200–2800 BCE. The Iranian Medes unified the country into the first of many empires in 625 BCE, after which it became the dominant cultural and political power in the region. Iran reached the pinnacle of its power during the Achaemenid Empire (First Persian Empire) founded by Cyrus the Great in 550 BCE, which at its greatest extent comprised major portions of the ancient world, stretching from parts of the Balkans (Bulgaria-Pannonia) and Thrace-Macedonia in the west, to the Indus Valley in the east, making it the largest empire the world had yet seen. The empire collapsed in 330 BCE following the conquests of Alexander the Great. The area eventually regained influence under the Parthian Empire and rose to prominence once more after the establishment of the Sasanian dynasty (Neo-Persian empire) in 224 CE, under which Iran again became one of the leading powers in the world along with the Byzantine Empire for the next four centuries.
Manichaeism and Zoroastrianism were largely replaced after Rashidun Muslims invaded Persia in 633 CE, and conquered it by 651 CE. Iran thereafter played a vital role in the subsequent Islamic Golden Age, producing numerous influential scientists, scholars, artists, and thinkers. The emergence in 1501 of the Safavid dynasty, which promoted the Twelver school of thought as the official religion, marked one of the most important turning points in Iranian and Muslim history. It also culminated into tensions, which in 1514 led to the Battle of Chaldiran. Starting in 1736 under Nader Shah, Iran would for the last time rise to high prominence, reaching its greatest territorial extent since the Sassanid Empire, and briefly possessing what was arguably the most powerful empire in the world. The Persian Constitutional Revolution of 1906 established the nation's first parliament, which operated within a constitutional monarchy. Following a coup d'état instigated by the UK and the US in 1953, Iran gradually became autocratic. Growing dissent against foreign influence and political repression culminated in the Iranian Revolution, which led to the establishment of an Islamic republic on 1 April 1979.
Tehran is the capital and largest city, serving as the cultural, commercial, and industrial center of the nation. Iran is a major regional and middle power, exerting considerable influence in international energy security and the world economy through its large reserves of fossil fuels, which include the largest natural gas supply in the world and the fourth-largest proven petroleum reserves. It hosts Asia's fourth-largest number of UNESCO World Heritage Sites. Globally, it ranks 8th in the world for the amount of books published per year, and in 2011 was ranked first in the world in the growth of its scientific output.
Iran is a founding member of the UN, NAM, OIC and OPEC. Its unique political system, based on the 1979 constitution, combines elements of a parliamentary democracy with a religious theocracy run by the country's clergy, wherein the Supreme Leader wields significant influence. A multicultural nation comprising numerous ethnic and linguistic groups, most inhabitants are Shi'ites, the Iranian rial is its currency, and Persian is the official language.