sluggishness vs lethargy

sluggishness and lethargy both are nouns.

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

As nouns, sluggishness and lethargy are synonyms defined as:

sluggishness and lethargy: inactivity; showing an unusual lack of energy

Other synonyms of sluggishness include flatness, languor, phlegm.

As nouns, sluggishness and lethargy are synonyms defined as:

sluggishness and lethargy: a state of comatose torpor (as found in sleeping sickness)

Other synonyms of sluggishness include lassitude.

sluggishness (noun)	lethargy (noun)
inactivity; showing an unusual lack of energy	weakness characterized by a lack of vitality or energy
the pace of things that move relatively slowly	inactivity; showing an unusual lack of energy
a state of comatose torpor (as found in sleeping sickness)	a state of comatose torpor (as found in sleeping sickness)