Baku is the capital and largest city of Azerbaijan, as well as the largest city on the Caspian Sea and in the Caucasus region. Baku is 28 metres (92 ft) below sea level, which makes it the lowest lying national capital in the world and also the largest city in the world below sea level. Baku lies on the southern shore of the Absheron Peninsula, on the Bay of Baku. Baku's urban population was estimated at two million people as of 2009. Baku is the primate city of Azerbaijan—it is the sole metropolis in the country, and about 25% of all inhabitants of the country live in Baku's metropolitan area.

Azerbaijani or Azeri, also referred to as Azeri Turkic, is a Turkic language from the Oghuz sub-branch. It is spoken primarily by the Azerbaijani people, who live mainly in the Republic of Azerbaijan, where the North Azerbaijani variety is spoken, and in the Azerbaijan region of Iran, where the South Azerbaijani variety is spoken. North Azerbaijani has official status in the Republic of Azerbaijan and Dagestan, but South Azerbaijani does not have official status in Iran, where the majority of Azerbaijani people live. Azerbaijani is also spoken to lesser varying degrees in Azerbaijani communities of Georgia and Turkey and by diaspora communities, primarily in Europe and North America.