appendix vs postscript

appendix and postscript both are nouns.

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

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

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

Other hypernyms of appendix include addendum, supplement.

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