The Rhine is a river that flows from Grisons in the eastern Swiss Alps to the North Sea coast in the Netherlands and is the twelfth longest river in Europe, at about 1,233 km (766 mi), with an average discharge of more than 2,000 m³/s (71,000 cu ft/s).

The Rhine and the Danube formed most of the northern inland frontier of the Roman Empire and, since those days, the Rhine has been a vital and navigable waterway carrying trade and goods deep inland. It has also served as a defensive feature and has been the basis for regional and international borders. The many castles and prehistoric fortifications along the Rhine testify to its importance as a waterway. River traffic could be stopped at these locations, usually for the purpose of collecting tolls, by the state that controlled that portion of the river.

The English name of the Rhine derives from Old English Rīn, which descends from Proto-Germanic *Rīnaz. This is also the source of the name in the other Germanic languages such as Dutch Rijn (formerly also Rhijn), German Rhein, Romansh Rain (via German) and also French Rhin and Spanish Rin, which came into the language through Old Frankish. This in turn derives from Indo-European *Reynos, from the root *rey- "to flow, to run", which is also the root of words like river and run. The Celtic/Gaulish name for the Rhine is Rēnos, which derives from the same Indo-European source as the Germanic name. It is also found in other names such as the Reno River in Italy, which got its name from Gaulish. The Latin name Rhēnus and Ancient Greek Ρήνος (Rhēnos) both derive from the Celtic word, and not from Indo-European directly, because they both share the change from -ei- to -ē-, which is characteristic of the Celtic languages but not of Latin or Greek (the Latin name would have been *Rīnus otherwise).