Archduke

Each of the six original Imperial domains has an archduke exercising overall control and acting as an intermediary between the Emperor and other levels of nobility (except for the Domain of Sylea, where the emperor is the archduke). Such nobles are referred to by the title archduke followed by the title of the domain, such as Archduke of Deneb. An archduke included a fief consisting of an entire world, generally retained as a private reserve.

Source: Imperial Encyclopedia