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.
常用漢字 are the commonly used kanji approved by the Japanese government, while 制限外の字 are those outside such official lists and may be restricted in certain contexts.
人名用漢字 are kanji specifically permitted for use in personal names, whereas 制限外の字 are those not allowed even for names.
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.