The subject of this article has not yet been localised outside of Japan by Konami. Please remember that these romanisations are non-canon.
The term itself referrs in general to any and all books written about or from the Old World. To prevent the spread of knowledge about the Terrus Firma or details about the founding of the Holy Empire of Ionia, there is a mandate for such books to be recovered by the Holy Imperial Army.
Such books are strictly managed by the Palace of All Things so as not to be seen by the common people.