second thought vs turnabout

second thought and turnabout both are nouns.

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

As nouns, turnabout is a hypernym of second thought; that is, turnabout is a word with a broader meaning than second thought:

Other hypernyms of second thought include change of mind, flip-flop, reversal, turnaround.

second thought (noun)	turnabout (noun)
thinking again about a choice previously made	turning in the opposite direction
	a decision to reverse an earlier decision