shoreline vs boundary

shoreline and boundary both are nouns.

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

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

Other hypernyms of shoreline include bound, bounds.

shoreline (noun)	boundary (noun)
a boundary line between land and water	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