flatness vs inactivity

flatness and inactivity both are nouns.

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

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

Other hypernyms of flatness include inactiveness, inertia.

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