Archduke

The title of Archduke (German Erzherzog) was invented in the Privilegium Maius, a forgery initiated by Duke Rudolf IV of Austria. Originally, it was meant to denote the ruler of the Archduchy of Austria, in any effort to put that ruler on par with the electorshipss, as Austria had been passed over in the Golden Bull of 1356, where the electorships had been assigned. Emperor Charles IV refused to recognize the title.
Duke Ernest the Iron and his descendants unilaterally assumed the title "Archduke."
This title was only officially recognized in 1453 by Emperor Frederick III, when the Habsburgs had (permanently) gained control of the office of the Holy Roman Emperor.

From the 16th century onward, Archduke or its female form, Archduchess, came to be used by all the members of the House of Habsburg, similar to the title Prince in many other royal houses. For example, Queen Marie Antoinette of France was born an Archduchess of Austria. This practice was maintained in the Austrian Empire (1804-1867) and the Austro-Hungarian Empire (1867-1918).

With the abolition of the monarchy, titles and the peerage system were also abolished in Austria. Thus, those members of the extended Habsburg family who are citizens of the Republic of Austria, are simply known by their respective first name and their surname Habsburg-Lothringen. The use of aristocratic titles such as archduke is in fact illegal in Austria. However, some members of the family who are citizens of other countries such as Germany, where aristocratic titles have become part of the name, may use the title.

Note that Archduke (Erzherzog) is a title distinct from Grand Duke (Grossherzog) used in the Russian and some other German royal houses.