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 inside 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 possessed 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.