add-on vs afterthought

add-on and afterthought both are nouns.

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

As nouns, afterthought is a hyponym of add-on; that is, afterthought is a word with a more specific, narrower meaning than add-on:

Other hyponyms of add-on include additive, annex, annexe, extension, wing, attachment, elongation, extension.

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