addendum vs postscript

addendum and postscript both are nouns.

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

As nouns, addendum and postscript are synonyms defined as:

addendum and postscript: textual matter that is added onto a publication; usually at the end

Other synonyms of addendum include supplement.

addendum (noun)	postscript (noun)
textual matter that is added onto a publication; usually at the end	a note appended to a letter after the signature
	textual matter that is added onto a publication; usually at the end