The 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, but 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.