back talk vs rejoinder

back talk and rejoinder both are nouns.

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

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

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

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

back talk (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