1. A historical region in present-day northwest Iran and Azerbaijan. It was settled by the Medes before the 8th century BC and later became a province of the Persian Empire. It became a separate kingdom after the death of Alexander the Great, but the region was much disputed and eventually conquered by Arabs (7th century), Turks (11th century), and Mongols (13th century). Persia controlled the region after c. 1605.
2. A country of Transcaucasia north of Iran on the Caspian Sea. It constitutes the northern part of the historical region of Azerbaijan, which was ceded to Russia by Persia in 1813 and 1828. Declared a Soviet republic in 1920, Azerbaijan was a part of the USSR from 1922 to 1991. Baku is the capital.