inaction vs inactivity

inaction and inactivity both are nouns.

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

As nouns, inaction and inactivity are synonyms defined as:

Other synonyms of inaction include inactiveness.

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