The Danube was once a long-standing frontier of the Roman Empire, and today flows through 10 countries, more than any other river in the world. Originating in Germany, the Danube flows southeast for 2,860 km (1,780 mi), passing through or touching the border of Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine before emptying into the Black Sea. Its drainage basin extends into nine more countries. The Danube river basin is the most biodiverse region in Europe, and is home to hundreds of 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.
Old European river name derived from a Proto-Indo-European *dānu. Other river names from the same root include the Dunaj, Dzvina/Daugava, Don, Donets, Dnieper, Dniestr, Dysna, Tana and Tuoni. In Rigvedic Sanskrit, dānu means "fluid, drop", in Avestan, the same word means "river". In the Rigveda, Dānu once appears as the mother of Vrtra, "a dragon blocking the course of the rivers." The Finnish word for Danube is Tonava, which is most likely derived from the word for the river in Swedish and German, Donau. Up North there is also a river called Tana. Its Sámi name Deatnu means "Great River". It is possible that dānu in Scythian as in Avestan was a generic word for "river": Dnieper and Dniestr, from Danapris and Danastius, are presumed to continue Scythian *dānu apara "far river" and *dānu nazdya- "near river", respectively.
Known to the ancient Greeks as the Istros (Ἴστρος) a borrowing from a Daco-Thracian name meaning "strong, swift" (akin to Sanskrit iṣiras "swift"). In Latin, the Danube was variously known as Danubius, Danuvius or as Ister. The Dacian/Thracian name was Donaris for the upper Danube and Istros for the lower Danube. The Thraco-Phrygian name was Matoas, "the bringer of luck".
The Latin name is masculine, as are all its Slavic names, except Slovenian, (the name of the Rhine is also masculine in Latin, most of the Slavic languages, as well as in German). The German Donau (Early Modern German Donaw, Tonaw, Middle High German Tuonowe) is feminine, as it has been re-interpreted as containing the suffix -ouwe "wetland".
The modern languages spoken in the Danube basin all use names related to Dānuvius: German: Donau (; (Austro-Bavarian: Doana); Silesian: Důnaj; Upper Sorbian: Dunaj; Slovak: Dunaj (; Hungarian: Duna (); Serbo-Croatian: Dunav / Дунав ( or Romanian: Dunărea (); Bulgarian: Дунав, Dunav (); Ukrainian: Дунай, Dunai (); Czech: Dunaj (Czech pronunciation: ); Polish: Dunaj (Polish pronunciation: ); Slovene: Donava (); Turkish: Tuna; Portuguese: Danúbio (); French: Danube ); Italian: Danubio (); Spanish: Danubio (); Romansh: Danubi.