appendix vs addendum

appendix and addendum both are nouns.

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

As nouns, addendum is a hypernym of appendix; that is, addendum is a word with a broader meaning than appendix:

appendix: supplementary material that is collected and appended at the back of a book
addendum: textual matter that is added onto a publication; usually at the end

Other hypernyms of appendix include postscript, supplement.

appendix (noun)	addendum (noun)
a vestigial process that extends from the lower end of the cecum and that resembles a small pouch	textual matter that is added onto a publication; usually at the end
supplementary material that is collected and appended at the back of a book