The Universe of the Four gods- This is a book written by Einosuuke Okuda. He wrote it unaware that it had a magical power to take in people insie the book and act as characters in the story. The book can tell what is happening to the characters.
Seven Star warriors / Seishi/ Seven Celestial Warriors- These are the warriors of a specific beast god, there are 28 in all. There are four gods they serve under and seven of them are serving under a god. They fight for their priestess, and if the priestess does not appear and they're already dead , they will just be reborn again and again until the priestess comes. You can identify them with the constellations marked somewhere on their bodies.
The four demon gods- The four demon gods are characters in the book or the real world who have powers, and they are possesed by a demon and they become gods by their own insanity.
Fushigi yuugi- The title of the series.
Fushigi yuugi Genbu Kaiden- a prequel story of Fushigi yuugi that tells more details about The Universe of the Four gods , except this is the story of the priestess of Genbu, Takiko Okuda.