noun
codicil
A legal document that supplements or modifies an existing will. Used in formal legal contexts.
遺言補足書を作成して、遺言の一部を変更した。
I made a codicil and changed part of the will.
遺言 is a will, while 遺言補足書 is a supplementary document that amends or adds to an existing will.
Compound of 遺言 (will) + 補足 (supplement) + 書 (document).