afterthought vs add-on

afterthought and add-on both are nouns.

Word	Noun	Adjective	Verb	Adverb
afterthought	Yes	No	No	No
add-on	Yes	No	No	No

As nouns, add-on is a hypernym of afterthought; that is, add-on is a word with a broader meaning than afterthought:

Other hypernyms of afterthought include addition, improver.

afterthought (noun)	add-on (noun)
an addition that was not included in the original plan	a component that is added to something to improve it
thinking again about a choice previously made	a supplementary component that improves capability
	a software module that extends or enhances the capabilities of an existing application