back saw vs backsaw

back saw and backsaw both are nouns.

Word	Noun	Adjective	Verb	Adverb
back saw	Yes	No	No	No
backsaw	Yes	No	No	No

As nouns, back saw and backsaw are synonyms defined as:

back saw and backsaw: a handsaw that is stiffened by metal reinforcement along the upper edge

back saw (noun)	backsaw (noun)
a handsaw that is stiffened by metal reinforcement along the upper edge	a handsaw that is stiffened by metal reinforcement along the upper edge