back talk vs comeback

back talk and comeback both are nouns.

Word	Noun	Adjective	Verb	Adverb
back talk	Yes	No	No	No
comeback	Yes	No	No	No

As nouns, comeback is a hypernym of back talk; that is, comeback is a word with a broader meaning than back talk:

back talk: an impudent or insolent rejoinder
comeback: a quick reply to a question or remark (especially a witty or critical one)

Other hypernyms of back talk include counter, rejoinder, replication, retort, return, riposte.

back talk (noun)	comeback (noun)
an impudent or insolent rejoinder	return by a celebrity to some previously successful activity
	a quick reply to a question or remark (especially a witty or critical one)