Diet (Fr. diete), a term applied to several political bodies of mediaeval and modern Europe, corresponding to the parliament in Great Britain, the cortes in Spain and Portugal, the states general, national assembly, and chambers in the history of France, and the congress in the United States. The derivation of the term from the Latin dies, day, as meaning a day fixed for the national deliberations on public affairs, is proved by the corresponding words in German (Reichstag), Dutch (Rijksdag), Swedish (Riksdag), and Danish (Rigsdag), all of which mean day of the empire; and by the similar Swiss term for the Helvetian diet (Tag-satzung). It is used by English and French historians of the state assemblies of the German empire and confederation, Poland, Hungary, Sweden, Switzerland, and some other countries, to which the Germans apply the distinctive appellations of Reichstag, Landtag, Landstande, Bundestag, Tagsatzung, etc. The diet of the German empire, which must not be confounded with the popular assemblies of the Germanic nations in the Carlovingian times, or with the assembly (Bundestag) of the German confederation as established by the congress of Vienna, had its rise after the dissolution of the Frankish empire, and was slowly developed under the successive German houses, undergoing material changes, particularly in the reigns of the emperor Charles IV. in the 14th century, Frederick III. in the 15th, and Charles V. in the 16th, until it received its ultimate modifications by the treaty of Westphalia in 1648, and the session of Ratisbon in 1663. From this date down to the dissolution of the empire in 1806, Ratisbon became its permanent seat, while in previous times the emperor had the privilege of choosing the place of its sessions.

The emperor, who formerly appeared in person, was now represented by a principal commissary, and all members of the empire by plenipotentiaries or agents. The diet consisted of three divisions, the so-called colleges of electors, princes, and imperial cities. The elector of Mentz, the archchancellor of the empire, presided in the electoral college, the archbishop of Salzburg and the archduke of Austria alternately in the college of princes, and the city where the session was held in that of the cities. The electors and cities had individual votes, as well as the chief members of the college of princes, while the imperial counts and imperial prelates, who belonged to the latter, had only collective votes by benches, of which there were four of counts and two of prelates. Resolutions were passed by majority, except in religious matters and those concerning individual members of the empire alone. But the concurrence of all the three colleges and the ratification of the emperor were required to establish a decree of the empire (Reichsschluss). Concurrence in case of difference of opinion was obtained by reconsideration and conference. The emperor had the right of rejection of the whole or a part of the bill, but not of modification.

The collection of resolutions passed and sanctioned by a diet was termed imperial recess (Reichs-abschied). The diet framed the laws of the empire, abolished and explained them, declared war and made peace, received and sent envoys, and concluded treaties. Imperial wars were proposed by the emperor, decided upon by majority, and carried on by the contingents of both the majority and the minority. The administration of the German confederation (Bund), which lasted, with some modifications, from 1815 to 1866, was vested in a diet (Bundestag or Bundesversammlung), the members of which were appointed by the various governments, Austria being the leading power, though the leadership came to be disputed by Prussia. (See Germany.) - The Polish diet (sejm) dates principally from the reign of Ladislas the Short, who in 1331 assembled all the nobles of his kingdom. Its form was established by law under Casimir IV. In the last period of independent Poland it was convened regularly every two years, for a session of no more than six weeks, twice successively in Warsaw and the third time at Grodno, in Lithuania. It consisted of a senate and a chamber of deputies (poset, plur. postowie). The latter were elected in previous municipal or district assemblies (sejmik, little diet). After the verification of their powers, the diet elected their president or marshal (marszatek). The initiation of measures was a royal prerogative, the sovereign furnishing a list of subjects to be discussed during the session.

In case of urgency the king could convoke an extraordinary diet to remain in session only two weeks. The most remarkable feature of the Polish diet was the so-called liberum veto, or the right of each member to prevent the enactment of a law or measure by individual opposition (nie pozwa-lam, I do not allow, or veto). This extreme of liberty, unknown in the history of any other nation, was remedied in part by confederations formed by the majority for the execution of its designs, and by timely application of violence, which silenced treacherous or bribed opponents; but it also led to fatal distractions, scenes of bloodshed, the permanence of factions, and ultimately, with other causes, to the fall of Poland. The diet of election was preceded by a diet of convocation, the arch-bishop of Gnesen, the primate of the state, having announced the vacancy of the throne. Hereupon all nobles appeared personally, assembling on the plain of Wola, near Warsaw, the senate in a shed (szopa), the common nobles in the kolo (circle). A diet of coronation, and, if that of election had been stormy, another of pacification, followed. - The diet of Hungary (dieta, or orszaggyules), formerly convened at various places, was from the time of the Turkish invasion held at Presburg. It consisted of two houses, the upper, or table of magnates, and the lower, or table of deputies.

In the latter, previous to the law of 1848, only the representatives of the nobles in the counties had a decisive personal vote. During the re-volutionary period of 1848-'9 the diet was held successively at Pesth, Debreczin, and Szegedin. Since 1861 it has its seat in Pesth. (See Hungary.)