extremity vs boundary

extremity and boundary both are nouns.

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

As nouns, boundary is a hyponym of extremity; that is, boundary is a word with a more specific, narrower meaning than extremity:

Other hyponyms of extremity include bound, bounds, end, terminal, extreme, extreme point, extremum, limit.

extremity (noun)	boundary (noun)
an external body part that projects from the body	the greatest possible degree of something
that part of a limb that is farthest from the torso	the line or plane indicating the limit or extent of something
the outermost or farthest region or point	a line determining the limits of an area
the greatest or utmost degree
an extreme condition or state (especially of adversity or disease)