lethargy vs inactiveness

lethargy and inactiveness both are nouns.

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

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

Other hypernyms of lethargy include inactivity, inertia.

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