impasse vs cul de sac

impasse and cul de sac both are nouns.

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

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

Other synonyms of impasse include blind alley, dead-end street.

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