As location of Nassau Castle and the former Stein and Heidenpütz castles, Nassau gave its name to the wider region surrounding it as well as to a prominent aristocratic dynasty, and through it, directly or indirectly to numerous other things worldwide, including: a sovereign state; a Prussian province; the Dutch royal house; the Luxembourgish ducal house; the capital of the Bahamas; one of the Cook Islands; and in the US, a county in Florida and a county in New York.

Au(e) derives from Middle High German ouwe, which is related to the Latin aqua ("water").

Administratively, Nassau is located in the Rhine-Lahn district (Rhein-Lahn-Kreis; seat: Bad Ems) in Rhineland-Palatinate (capital: Mainz).

Close to Nassau are two stations of the Cologne–Frankfurt high-speed rail line connecting Frankfurt and Wiesbaden to Bonn, Cologne, Düsseldorf and the Ruhr.

The earliest known surviving mention of Nassau refers to the Villa Nassova estate of the Bishopric of Worms in a 915 deed.

Nassau suffered heavily from bombings by American B-26 Marauders during World War II, all three hospitals and almost all of the inner city were completely destroyed.

The Grand Duke of Luxembourg still uses Nassau as a title and it is also part of the Dutch royal family (called Orange-Nassau).

[4] The Imperial Baron Heinrich Friedrich Karl vom und zum Stein, Prussian statesman and reformer, was born in Nassau on October 25, 1757.

The family estate lies in the centre of the town, called the Steinische Hof, which today is still in the possession of the descendants of the reformer, the Counts von Kanitz.