The Caucasus, or Caucasia, is a transcontinental region between the Black Sea and the Caspian Sea, mainly comprising Armenia, Azerbaijan, Georgia, and parts of Southern Russia. The Caucasus Mountains, including the Greater Caucasus range, have historically been considered as a natural barrier between Eastern Europe and West Asia.

The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake and sometimes referred to as a full-fledged sea. An endorheic basin, it lies between Europe and Asia: east of the Caucasus, west of the broad steppe of Central Asia, south of the fertile plains of Southern Russia in Eastern Europe, and north of the mountainous Iranian Plateau. It covers a surface area of 371,000 km2 (143,000 sq mi), an area approximately equal to that of Japan, with a volume of 78,200 km3 (19,000 cu mi). It has a salinity of approximately 1.2%, about a third of the salinity of average seawater. It is bounded by Kazakhstan to the northeast, Russia to the northwest, Azerbaijan to the southwest, Iran to the south, and Turkmenistan to the southeast.