back street vs alleyway

back street and alleyway both are nouns.

Word	Noun	Adjective	Verb	Adverb
back street	Yes	No	No	No
alleyway	Yes	No	No	No

As nouns, back street and alleyway are synonyms defined as:

Other synonyms of back street include alley.

back street (noun)	alleyway (noun)
a narrow street with walls on both sides	a narrow street with walls on both sides