flatness vs inactiveness

flatness and inactiveness both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatness	Yes	No	No	No
inactiveness	Yes	No	No	No

As nouns, inactiveness is a hypernym of flatness; that is, inactiveness is a word with a broader meaning than flatness:

Other hypernyms of flatness include inactivity, inertia.

flatness (noun)	inactiveness (noun)
the property of having two dimensions	a disposition to remain inactive or inert
inactivity; showing an unusual lack of energy	the state of being inactive
the property of having little or no contrast; lacking highlights or gloss
a deficiency in flavor
a want of animation or brilliance