noun
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.
この名前には制限外の字が含まれているため、戸籍に登録できません。
This name contains a forbidden kanji, so it cannot be registered in the family register.
制限外の字を使うと、パスポートの申請が却下されることがあります。
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.