Durchlaucht

His/Her Serene Highness (abbreviation: HSH, oral address: Your Serene Highness) is a style used today by the reigning families of Liechtenstein and Monaco. Until 1918, it was also associated with the princely titles of members of some German ruling and mediatised dynasties and with a few princely but non-ruling families. It was also the form of address used for cadet members of the dynasties of France, Italy, Russia and Ernestine Saxony, under their monarchies. Additionally, the treatment was granted for some, but not all, princely yet non-reigning families of Bohemia, Hungary, Italy, Poland, Romania and Russia by emperors or popes.

In a number of older English dictionaries, serene as used in this context means supreme; royal; august; marked by majestic dignity or grandeur; or high or supremely dignified. The style Serene Highness, as a manner of address, has an antiquity lower to that of Highness throughout Europe, excluding the Latin language countries.

The current, legal usage of the style in the German-speaking countries is confined to the Princely Family of Liechtenstein, the entirety of which bears the treatment.

The German term is Durchlaucht, a translation of the Latin (su)perillustris. This is usually translated into English as Serene Highness, however, it would be more literal to translate it as superior to, above, beyond or greater than illustrious, as it is an augmentation of Erlaucht ("illustrious"), which was accorded to immediate counts (Reichsgrafen) of the Holy Roman Empire and by mediatised counts of the German Confederation and the German Empire. The 1911 edition of the Encyclopædia Britannica wryly observes that a perfectly logical English version might be "Your Transparency".

...
Wikipedia