back talk vs backtalk

back talk and backtalk both are nouns.

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

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

Other synonyms of back talk include lip, mouth, sass, sassing.

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