backtalk vs back talk

backtalk and back talk both are nouns.

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

As nouns, backtalk and back talk are synonyms defined as:

Other synonyms of backtalk include lip, mouth, sass, sassing.

backtalk (noun)	back talk (noun)
an impudent or insolent rejoinder	an impudent or insolent rejoinder