a republic in SW Asia, between the Caspian Sea and the Persian Gulf: a monarchy until an Islamic revolution in 1979 headed by the Ayatollah Khomeini when the Shah was obliged to leave the country. Consists chiefly of a high central desert plateau almost completely surrounded by mountains, a semitropical fertile region along the Caspian coast, and a hot and dry area beside the Persian Gulf. Oil is the most important export. Official language: Persian (Iranian or Farsi). Official religion: (Shiite) Muslim. Currency: rial. Capital: Tehran. Pop: 81 162 788 (2017 est). Area: 1 647 050 sq km (635 932 sq miles)