Donbass () or Donbas () is a historical, cultural, and economic region in eastern Ukraine. The word Donbass is a portmanteau formed from Donets Basin (, translit. Donetskyi basein; , Donetskiy bassein), which refers to the river Donets that flows through it. Multiple definitions of the region’s extent exist, though its boundaries have never been officially demarcated. The most common definition in use today refers to the Donetsk and Luhansk oblasts of Ukraine, whilst the historical coal mining region excluded parts of these oblasts, and included areas in Dnipropetrovsk Oblast and Southern Russia. A Euroregion of the same name is composed of Donetsk and Luhansk oblasts in Ukraine and Rostov Oblast in Russia. Donbass formed the historical border between the Zaporizhian Sich and Don Cossack Host. It has been an important coal mining area since the late 19th century, when it became a heavily industrialised territory. It suffers from urban decay and industrial pollution. In March 2014, following the 2014 Ukrainian revolution, large swaths of the Donbass became gripped by unrest. This unrest later grew into a war between pro-Russian separatists affiliated with the Donetsk and Luhansk People’s Republics, and the post-revolutionary Ukrainian government. Until the ongoing war, the Donbass was the most densely populated of all the regions of Ukraine apart from the capital city of Kiev. Before the war, the city of Donetsk (then the fifth largest city of Ukraine) was considered the unofficial capital of the Donbass. Large cities (over 100,000 inhabitants) also included Luhansk, Mariupol, Makiivka, Horlivka, Kramatorsk, Sloviansk, Alchevsk, Sievierodonetsk and Lysychansk. Now the city of Kramatorsk is the interim administrative center of the Donetsk Oblast, whereas the interim center of Luhansk Oblast is the city of Severodonetsk. On the separatist side, Donetsk, Makiivka, and Horlivka are now the largest cities in the Donetsk People’s Republic, and Luhansk and Alchevsk in the Lugansk People’s Republic.