lethargy vs inactivity

lethargy and inactivity both are nouns.

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

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

Other hypernyms of lethargy include inactiveness, inertia.

lethargy (noun)	inactivity (noun)
weakness characterized by a lack of vitality or energy	being inactive; being less active
inactivity; showing an unusual lack of energy	a disposition to remain inactive or inert
a state of comatose torpor (as found in sleeping sickness)	the state of being inactive