outer boundary vs boundary

outer boundary and boundary both are nouns.

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

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

Other hypernyms of outer boundary include bound, edge.

outer boundary (noun)	boundary (noun)
the outside boundary or surface of something	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