A Gau (plural Gaue) is a German term for a region within a country, often a former or actual province. It was used in medieval times, when it can be seen as roughly corresponding to an English shire, and was revived as an administrative subdivision during the period of Nazi rule in Germany.
In the German-speaking lands east of the Rhine, the Gau formed the unit of administration of the Carolingian empire during the ninth and tenth centuries. Many such a territory evolved into what would become known as a Grafschaft, the territory of a Graf or count; the count was originally an appointed governor, but the position became in time a hereditary vassal princedom, or fief.
By 1938 all of Germany was divided into around thirty Gaue. Following the suppression of the political institutions of the Länder (states) in 1934, the Gaue had become the de facto administrative region of government, and each individual Gauleiter had considerable power within his territory.
With Germany's annexation of neighbouring territories beginning in the late 1930s, a new unit of civil administration, the Reichsgau, was also created. After the successful invasion of France in 1940, the Alsace-Lorraine was re-annexed by Germany. The former département of Moselle was incorporated into the Gau of Saar-Palatinate, while Bas-Rhin and Haut-Rhin were incorporated into Baden Gau. Similarly, the formerly independent state of Luxembourg was annexed to Koblenz-Trier, and the Belgian territories of Eupen and Malmedy were incorporated into Cologne-Aachen.
German-speaking territories annexed to Germany from 1938 were generally organised into Reichsgaue. Unlike the pre-existing Gaue, the new Reichsgaue formally combined the spheres of both party and civil administration.
Following the annexation of Austria in 1938, the country, briefly renamed "Ostmark", was sub-divided into seven Reichsgaue. These had boundaries broadly the same as the former Austrian Länder (states), with the Tyrol and Vorarlberg being merged as "Tyrol-Vorarlberg", Burgenland being divided between Styria and "Lower Danube" (the re-named Lower Austria). Upper Austria was also re-named "Upper Danube", thus eliminating the name of "Austria" from the official map. A small number of boundary changes were also made, the most significant of which was the massive expansion of Vienna's official territory, at the expense of "Lower Danube".
Northern and eastern territory annexed from the dismembered Czechoslovakia were mainly organised as the Reichsgau of Sudetenland, with territory to the south annexed to the Reichsgaue of Lower and Upper Danube.
Following the invasion of Poland in 1939, territories lost at the Treaty of Versailles, together with some adjacent territory, were re-annexed to Germany as the Reichsgaue of Danzig-Westpreussen (which also incorporated the former Free City of Danzig) and Wartheland.
Notably, the German translation of The Lord of the Rings opted not to use Gau for the translation of the Shire, due to its Nazi associations.