exterior angle vs external angle

exterior angle and external angle both are nouns.

Word	Noun	Adjective	Verb	Adverb
exterior angle	Yes	No	No	No
external angle	Yes	No	No	No

As nouns, exterior angle and external angle are synonyms defined as:

exterior angle and external angle: the supplement of an interior angle of a polygon

exterior angle (noun)	external angle (noun)
the supplement of an interior angle of a polygon	the supplement of an interior angle of a polygon