Azerbaijan, officially Azerbaijani Republic, republic in the Transcaucasia region of western Asia, bordered on the north by Russia, on the north-west by Georgia, on the east by the Caspian Sea, on the south by Iran, and on the west by Armenia. Formerly a republic of the Union of Soviet Socialist Republics (USSR), the republic includes the regions known under the Soviets as the Nagorno-Karabakh Autonomous Oblast and the Nakhichevan Autonomous Soviet Socialist Republic (ASSR), which is separated from the rest of Azerbaijan by a mountainous strip in Armenia. Baku, a large port city on the Caspian Sea, is the capital and largest city.