backtalk vs rejoinder

backtalk and rejoinder both are nouns.

Word	Noun	Adjective	Verb	Adverb
backtalk	Yes	No	No	No
rejoinder	Yes	No	No	No

As nouns, rejoinder is a hypernym of backtalk; that is, rejoinder is a word with a broader meaning than backtalk:

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

Other hypernyms of backtalk include comeback, counter, replication, retort, return, riposte.

backtalk (noun)	rejoinder (noun)
an impudent or insolent rejoinder	a quick reply to a question or remark (especially a witty or critical one)
	(law) a pleading made by a defendant in response to the plaintiff's replication