austereness vs plainness

austereness and plainness both are nouns.

Word	Noun	Adjective	Verb	Adverb
austereness	Yes	No	No	No
plainness	Yes	No	No	No

As nouns, plainness is a hypernym of austereness; that is, plainness is a word with a broader meaning than austereness:

austereness (noun)	plainness (noun)
extreme plainness	the appearance of being plain and unpretentious
	an appearance that is not attractive or beautiful
	clarity as a consequence of being perspicuous
	the state of being unmixed with other material