Iran,[a] officially the Islamic Republic of Iran (IRI)[c] and also known as Persia,[d] is a country in West Asia. It borders Turkey to the northwest and Iraq to the west, Azerbaijan, Armenia, the Caspian Sea, and Turkmenistan to the north, Afghanistan to the east, Pakistan to the southeast, the Gulf of Oman and the Persian Gulf to the south. With a multi-ethnic population of nearly 86 million in an area of 1,648,195 km2 (636,372 sq mi), Iran ranks 17th globally in both geographic size and population. It is the sixth-largest country entirely in Asia and one of the world's most mountainous countries. Officially an Islamic republic, Iran is divided into five regions with 31 provinces. Tehran is the nation's capital, largest city and financial centre.