The Collaborative International Dictionary of English v.0.48:

Diet \Di"et\, n. [F. di[`e]te, LL. dieta, diaeta, an assembly, a
   day's journey; the same word as diet course of living, but
   with the sense changed by L. dies day: cf. G. tag day, and
   Reichstag.]
   A legislative or administrative assembly in Germany, Poland,
   and some other countries of Europe; a deliberative
   convention; a council; as, the Diet of Worms, held in 1521.
   Specifically: Any of various national or local assemblies;
   as,
   (a) Occasionally, the Reichstag of the German Empire,
       Reichsrath of the Austrian Empire, the federal
       legislature of Switzerland, etc.
   (b) The legislature of Denmark, Sweden, Japan, or Hungary.
   (c) The state assembly or any of various local assemblies in
       the states of the German Empire, as the legislature
       (Landtag) of the kingdom of Prussia, and the Diet of the
       Circle (Kreistag) in its local government.
   (d) The local legislature (Landtag) of an Austrian province.
   (e) The federative assembly of the old Germanic Confederation
       (1815 -- 66).
   (f) In the old German or Holy Roman Empire, the great formal
       assembly of counselors (the Imperial Diet or Reichstag)
       or a small, local, or informal assembly of a similar kind
       (the Court Diet, or Hoftag).

   Note: The most celebrated Imperial Diets are the three
         following, all held under Charles V.:

   Diet of Worms, 1521, the object of which was to check the
      Reformation and which condemned Luther as a heretic;

   Diet of Spires, or Diet of Speyer, 1529, which had the
      same object and issued an edict against the further
      dissemination of the new doctrines, against which edict
      Lutheran princes and deputies protested (hence
      Protestants):

   Diet of Augsburg, 1530, the object of which was the
      settlement of religious disputes, and at which the
      Augsburg Confession was presented but was denounced by the
      emperor, who put its adherents under the imperial ban.
      [Webster 1913 Suppl.]