knife-edge vs boundary

knife-edge and boundary both are nouns.

Word	Noun	Adjective	Verb	Adverb
knife-edge	Yes	No	No	No
boundary	Yes	No	No	No

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

Other hypernyms of knife-edge include bound, limit.

knife-edge (noun)	boundary (noun)
a narrow 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