flatness vs lethargy

flatness and lethargy both are nouns.

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

As nouns, flatness and lethargy are synonyms defined as:

Other synonyms of flatness include languor, phlegm, sluggishness.

flatness (noun)	lethargy (noun)
the property of having two dimensions	weakness characterized by a lack of vitality or energy
inactivity; showing an unusual lack of energy	inactivity; showing an unusual lack of energy
the property of having little or no contrast; lacking highlights or gloss	a state of comatose torpor (as found in sleeping sickness)
a deficiency in flavor
a want of animation or brilliance