lethargy vs flatness

lethargy and flatness both are nouns.

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

As nouns, lethargy and flatness are synonyms defined as:

Other synonyms of lethargy include languor, phlegm, sluggishness.

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