afterthought vs addition

afterthought and addition both are nouns.

Word	Noun	Adjective	Verb	Adverb
afterthought	Yes	No	No	No
addition	Yes	No	No	No

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

Other hypernyms of afterthought include add-on, improver.

afterthought (noun)	addition (noun)
an addition that was not included in the original plan	the arithmetic operation of summing; calculating the sum of two or more numbers
thinking again about a choice previously made	the act of adding one thing to another
	a component that is added to something to improve it
	a suburban area laid out in streets and lots for a future residential area
	something added to what you already have
	a quantity that is added