Terminology

Furyoku
"Furyoku" is a term used to determine the measure of sixth sense a person possesses. This can be increased by having a near-death experience such as when Yoh went into the dark cave to train. More Furyoku means more control over one's spirit and more powerful forms of oversoul. With this power, a shaman can produce an over soul. In the English manga, it is "Mana".
 * Furyoku (巫力)

Manjien
The "Mantannian Dictionary" is a "Dictionary/ Encyclopedia/ Compendium of all human knowledge" that Manta frequently consults to find information on people, cultures, and other subjects. The book's Japanese name blends "Manta" with the "Kōjien" (広辞苑) Japanese dictionary. In the English anime, the book is either not given a name or it is called a "ghost story book" or "book about shamans" that Morty carries around.
 * Manjien (万辞苑 Manjien) / Mantannian Dictionary (English manga only)

Seirei-class
The classification for spirits who have existed for 500 years or more and whose souls have shed their physical attachment to achieve their purest form. Sacred--or 'higher'--spirits are more appropriately ghosts who have ascended beyond their ego, enabling them to assume any form or shape freely. After this "ascending", the spirit does not have to become stronger since this is the highest form of power. Only the shaman has to train and grow stronger.
 * Seirei-class ('Sacred Souls' in English Anime)