appendix vs codicil

appendix and codicil both are nouns.

Word	Noun	Adjective	Verb	Adverb
appendix	Yes	No	No	No
codicil	Yes	No	No	No

As nouns, codicil is a hyponym of appendix; that is, codicil is a word with a more specific, narrower meaning than appendix:

appendix: supplementary material that is collected and appended at the back of a book
codicil: a supplement to a will; a testamentary instrument intended to alter an already executed will

Other hyponyms of appendix include shirttail.

appendix (noun)	codicil (noun)
a vestigial process that extends from the lower end of the cecum and that resembles a small pouch	a supplement to a will; a testamentary instrument intended to alter an already executed will
supplementary material that is collected and appended at the back of a book