afterthought vs second thought

afterthought and second thought both are nouns.

Word	Noun	Adjective	Verb	Adverb
afterthought	Yes	No	No	No
second thought	Yes	No	No	No

As nouns, afterthought and second thought are synonyms defined as:

afterthought and second thought: thinking again about a choice previously made

Other synonyms of afterthought include reconsideration, rethink.

afterthought (noun)	second thought (noun)
an addition that was not included in the original plan	thinking again about a choice previously made
thinking again about a choice previously made