Domain of Dread
Domain of Dread is the official title given in out-of-universe sources to the individual pocket-realms that collectively make up the Demiplane of Dread. Centered around a figure of noteworthy spiritual corruption and evil called a Darklord, the Demiplane is made of myriad Domains of Dread spread throughout its mists. Isolated, lone Domains are commonly known as "Islands of Terror". Groups of Domains in physical conjunction with each other are called "Clusters". The metaphorical center of the Ravenloft setting, called simply "The Core" in-universe, is a massive array of Domains, most of them the size of small countries.