Definition of "Aar"

A river in Switzerland, flowing 292 km from the glaciers of the Bernese Alps into the Rhine at Koblenz, on the Swiss–German border.  examples

A river in Germany that flows 50 km from the Taunus in Hesse into the Lahn at Diez, Rhineland-Palatinate.  examples

A tributary in Hesse, Germany of the Dill river, itself a tributary of the Lahn.  examples