In politics, a diet (//, //) is a formal deliberative assembly. The term is used historically for deliberative assemblies such as the German Imperial Diet (the general assembly of the Imperial Estates of the Holy Roman Empire), as well as a designation for modern-day legislative bodies of certain countries and states such as the National Diet of Japan, or the German Bundestag, the Federal Diet.
The term (also in the nutritional sense) might be derived from Medieval Latin dieta, meaning both "parliamentary assembly" and "daily food allowance", from earlier Latin diaeta, possibly from the Greek διαιτησία (= arbitration), or transcribing Classical Greek δίαιτα diaita, meaning "way of living", and hence also "diet", "regular (daily) work".
Through a false etymology, reflected in the spelling change replacing ae with e, the word "diet" came to be associated with Latin dies, "date". It came to be used in postclassical Europe in the sense of "an assembly" because of its use for the work of an assembly meeting on a daily basis or a given day of the time period, and hence for the assembly itself. The association with dies is reflected in the German language's use of Tagung (meeting) and -tag (not only meaning "day", as in Montag—Monday—but also "parliament", "council", or other law-deliberating chamber, as in Bundestag or Reichstag).
In this sense, it commonly refers to the Imperial Diet assemblies of the Holy Roman Empire:
After the Second Peace of Thorn of 1466, a German-language Prussian diet Landtag was held in the lands of Royal Prussia, a province of Poland in personal union with the king of Poland.
The Croatian word for a legislative assembly is sabor (from the verb sabrati se, "to assemble"); in historic contexts it is often translated with "diet" in English, as in "the Diet of Dalmatia" (Dalmatinski sabor), "the Croatian Diet" (Hrvatski sabor), "the Hungarian-Croatian Diet" (Ugarsko-hrvatski sabor), or Diet of Bosnia ("Bosansko-hercegovački sabor").
The Hungarian Diet, customarily called together every three years in Székesfehérvár, Buda or Pressburg, was also called "Diéta" in the Habsburg Empire before the 1848 revolution.
The Riksdag of the Estates was the diet of the four estates of Sweden, from the 15th century until 1866. The Diet of Finland was the successor to the Riksdag of the Estates in the Grand Duchy of Finland, from 1809 to 1906.
The Swiss legislature was the Tagsatzung (French: Diète) before the Federal Assembly replaced it in the mid-19th century.