sluggishness vs flatness

sluggishness and flatness both are nouns.

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

As nouns, sluggishness and flatness are synonyms defined as:

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

Other synonyms of sluggishness include languor, lethargy, phlegm.

sluggishness (noun)	flatness (noun)
inactivity; showing an unusual lack of energy	the property of having two dimensions
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)	the property of having little or no contrast; lacking highlights or gloss
	a deficiency in flavor
	a want of animation or brilliance