cul de sac vs impasse

cul de sac and impasse both are nouns.

Word	Noun	Adjective	Verb	Adverb
cul de sac	Yes	No	No	No
impasse	Yes	No	No	No

As nouns, cul de sac and impasse are synonyms defined as:

Other synonyms of cul de sac include blind alley, dead-end street.

cul de sac (noun)	impasse (noun)
a street with only one way in or out	a street with only one way in or out
a passage with access only at one end	a situation in which no progress can be made or no advancement is possible