second thought vs afterthought

second thought and afterthought both are nouns.

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

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

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

Other synonyms of second thought include reconsideration, rethink.

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