rejoinder vs back talk

rejoinder and back talk both are nouns.

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

As nouns, back talk is a hyponym of rejoinder; that is, back talk is a word with a more specific, narrower meaning than rejoinder:

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

Other hyponyms of rejoinder include backtalk, lip, mouth, sass, sassing.

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