The Danube basin and the Danube Region in which Serbia takes part
Background
The European Commission describes the area covered by the EU Strategy for the Danube Region as mainly the basin of the 2.857 km long Danube River, including also the parts of the mountain ranges where its tributaries originate, such as the Alps and the Carpathians. It stretches from the Black Forest (Germany) to the Black Sea (Romania-Moldova-Ukraine) and is home to around 115 million inhabitants. Involving 14 countries, it is the largest and most diverse macro-regional strategy: nine EU Member States (Austria, Bulgaria, Croatia, Czechia, parts of Germany, Hungary, Romania, Slovakia, Slovenia), three Accession Countries (Bosnia and Herzegovina, Montenegro, Serbia) and Ukraine and Moldova. The Danube Strategy was formally created on 13 April 2011 when the EU Council endorsed the Communication and the accompanying Action Plan. The Commission lists water pollution, floods and climate change among the environmental threats the region faces, alongside shipping potential that is not fully used and gaps in road and rail links. Note that the Commission's page writes the river's length as 2.857 km, using a full stop where English would use a comma as the thousands separator.
In plain words
EUSDR EU Strategy for the Danube Region — The European Union's coordination framework for the countries of the Danube basin.
Macro-regional strategy EU cooperation framework for a shared geographic region — A framework that brings member states and non-member neighbours together around one geography, without extra EU money of its own.