outer boundary vs periphery

outer boundary and periphery both are nouns.

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

As nouns, outer boundary and periphery are synonyms defined as:

outer boundary and periphery: the outside boundary or surface of something

Other synonyms of outer boundary include fringe.

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