codicil

Etymology

Borrowed from Middle French codicille, from Latin cōdicillus, diminutive of cōdex. See code.

noun

1. (law) An addition or supplement that explains, modifies, or revokes a will or part of one.
2. An addition or supplement modifying any official document, such as a treaty.
   So insistent was this demand that the Wyandot actually received a codicil to the treaty […] 2004, Barbara Alice Mann, “The Greenville Treaty of 1795: Pen-and-Ink Witchcraft in the Struggle for the Old Northwest”, in Bruce E. Johansen, editor, Enduring Legacies: Native American Treaties and Contemporary Controversies, Praeger, page 155
   Those loose ends were tied up in a little-understood clarification of Brexit called the Northern Ireland protocol, ratified in January 2020. It looked like a mere codicil three years ago; now it looks like a serious diplomatic blunder that could threaten Britain’s territory and the region’s peace. 2023-01-26, Christopher Caldwell, “It’s Anyone’s Guess What Will Happen in Northern Ireland in the Next 12 Weeks”, in The New York Times, →ISSN
3. (by extension) Any appendix or addition.

verb

1. (transitive, intransitive) To add a codicil to something.