inactivity vs lethargy

inactivity and lethargy both are nouns.

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

As nouns, lethargy is a hyponym of inactivity; that is, lethargy is a word with a more specific, narrower meaning than inactivity:

Other hyponyms of inactivity include flatness, languor, phlegm, sluggishness, restfulness, passiveness, passivity, indolence, laziness.

inactivity (noun)	lethargy (noun)
being inactive; being less active	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)