The Danube (ˈdæn.juːb ; ) is the second-longest river in Europe, after the Volga in Russia. It flows through much of Central and Southeastern Europe, from the Black Forest into the Black Sea. A large and historically important river, it was once a frontier of the Roman Empire and today connects ten European countries, running through their territories or being a border. Originating in Germany, the Danube flows southeast for , passing through or bordering Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine. Among the many cities on the river are four national capitals: Vienna, Bratislava, Budapest, and Belgrade. Its drainage basin amounts to 817 000 km2 and extends into nine more countries.
The Danube's longest headstream Breg rises in Furtwangen im Schwarzwald, while the river carries its name from its source confluence in Donaueschingen onwards. Since ancient times, the Danube has been a traditional trade route in Europe. Today, of its total length are navigable. The Danube is linked to the North Sea via the Rhine–Main–Danube Canal, connecting the Danube at Kelheim with the Main at Bamberg. The river is also an important source of hydropower and drinking water.
The Danube river basin is home to fish species such as pike, zander, huchen, Wels catfish, burbot and tench. It is also home to a large diversity of carp and sturgeon, as well as salmon and trout. A few species of euryhaline fish, such as European seabass, mullet, and eel, inhabit the Danube Delta and the lower portion of the river.
Today the river carries its name from its source confluence in Donaueschingen onwards. Its longest headstream Breg rises in Furtwangen im Schwarzwald.
The river was known to the ancient Greeks as the Istros (Ἴστρος) from a root possibly also encountered in the ancient name of the Dniester (Danaster in Latin, Tiras in Greek) and akin to Iranic turos 'swift' and Sanskrit iṣiras (इषिरस्) 'swift', from the PIE , 'to flow'.