forbidden kanji
Refers to kanji that are not included in the official list of characters permitted for use in names, official documents, or certain publications. Often used in the context of Japanese naming regulations.
This name contains a forbidden kanji, so it cannot be registered in the family register.
Using a forbidden kanji may result in your passport application being rejected.
Compound of 制限 (restriction) + 外 (outside) + の (possessive particle) + 字 (character). The term is a straightforward description of kanji that fall outside official restrictions.