Reich (; German: [ˈʁaɪç] (listen)) is a German noun whose meaning is analogous to the English word "realm"; this is not to be confused with the German adjective "reich" which means "rich". The terms Kaiserreich (literally the "realm of an emperor") and Königreich (literally the "realm of a king") are respectively used in German in reference to empires and kingdoms. The Cambridge Advanced Learner's Dictionary indicates that in English usage, the term "the Reich" refers to "Germany during the period of Nazi control from 1933 to 1945".

The term Deutsches Reich (sometimes translated to "German Empire") continued to be used even after the collapse of the German Empire and the abolition of the monarchy in 1918. There was no emperor, but many Germans had imperialistic ambitions. According to Richard J. Evans:

The continued use of the term "German Empire", Deutsches Reich, by the Weimar Republic ... conjured up an image among educated Germans that resonated far beyond the institutional structures Bismarck created: the successor to the Roman Empire; the vision of God's Empire here on earth; the universality of its claim to suzerainty; and in a more prosaic but no less powerful sense, the concept of a German state that would include all German speakers in central Europe—"one People, one Reich, one Leader", as the Nazi slogan was to put it.

The term is derived from the Germanic word which generally means "realm", but in German, it is typically used to designate a kingdom or an empire, especially the Roman Empire. The terms Kaisertum ("Imperium") and Kaiserreich ("Imperial realm") are used in German to more specifically define an empire ruled by an emperor.

Reich is comparable in meaning and development (as well as descending from the same Proto-Indo-European root) to the English word realm (via French reaume "kingdom" from Latin regalis "royal"). It is used for historical empires in general, such as the Roman Empire (Römisches Reich), Persian Empire (Perserreich), and both the Tsardom of Russia and the Russian Empire (Zarenreich, literally "Tsars' realm"). Österreich, the name used for Austria today is composed of "Öster" and "Reich" which, literally translated, means "Eastern Realm". The name once referred to the Eastern parts of the Holy Roman Empire.

In the history of Germany specifically, it is used to refer to:

• the early medieval Frankish Realm (Francia) and Carolingian Empire (the Fränkisches Reich and Karolingerreich);
• the Holy Roman Empire (Heiliges Römisches Reich), which lasted from the coronation of Charlemagne as Holy Roman Emperor in 800, until 1806, when it was dissolved during the Napoleonic Wars;
• the German Empire (Deutsches Reich or Deutsches Kaiserreich), which lasted from the unification of Germany in 1871 until its collapse after World War I, during the German Revolution of 1918–1919;
• the Weimar Republic of 1919–1933 continued to use Deutsches Reich as its official name;
• Nazi Germany, the state often referred to as the Third Reich, which lasted from the Enabling Act in 1933 until the end of World War II in Europe in 1945. It continued to use the official name, Deutsches Reich, until 1943, when it was renamed to the Großdeutsches Reich (Greater German Empire).

The Nazis adopted the term "Third Reich" to legitimize their government as the rightful successor to the retroactively renamed "First" and "Second" Reichs – the Holy Roman Empire and the German Empire, respectively; the Nazis discounted the legitimacy of the Weimar Republic entirely. The terms "First Reich" and "Second Reich" are not used by historians, and the term "Fourth Reich" is mainly used in fiction and political humor, although it is also used by those who subscribe to neo-Nazism.