hairline vs boundary

hairline and boundary both are nouns.

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

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

Other hypernyms of hairline include bound, bounds.

hairline (noun)	boundary (noun)
the natural margin formed by hair on the head	the greatest possible degree of something
a very thin line	the line or plane indicating the limit or extent of something
	a line determining the limits of an area