ReichstagPronunciation: /ˈrʌɪxsˌtɑːg, ˈrʌɪks-/Definition of Reichstag the main legislature of the German state under the Second and Third Reichs. the building in which the main German legislature met, badly damaged by fire on the Nazi accession to power in 1933. Origin:German, from Reichs 'of the empire' + Tag 'diet' (see diet2)Reichstag in other Oxford dictionaries Definition of Reichstag in the US English dictionary