inactive vs inactivity

inactive is not a noun while inactivity is a noun.

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

inactive (noun)	inactivity (noun)
	being inactive; being less active
	a disposition to remain inactive or inert
	the state of being inactive

inactive (adjective)	inactivity (adjective)
(military) not involved in military operations
not active physically or mentally
(pathology) not progressing or increasing; or progressing slowly
lacking activity; lying idle or unused
lacking in energy or will
(of e.g. volcanoes) not erupting and not extinct
not exerting influence or change
not engaged in full-time work
not in physical motion
(chemistry) not participating in a chemical reaction; chemically inert