borderline vs boundary

borderline and boundary both are nouns.

Word	Noun	Adjective	Verb	Adverb
borderline	Yes	Yes	No	No
boundary	Yes	No	No	No

As nouns, boundary is a hypernym of borderline; that is, boundary is a word with a broader meaning than borderline:

Other hypernyms of borderline include bound, bounds.

borderline (noun)	boundary (noun)
a line that indicates a boundary	the greatest possible degree of something
	the line or plane indicating the limit or extent of something
	a line determining the limits of an area

borderline (adjective)	boundary (adjective)
of questionable or minimal quality