phlegm vs flatness

phlegm and flatness both are nouns.

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

As nouns, phlegm and flatness are synonyms defined as:

Other synonyms of phlegm include languor, lethargy, sluggishness.

phlegm (noun)	flatness (noun)
inactivity; showing an unusual lack of energy	the property of having two dimensions
expectorated matter; saliva mixed with discharges from the respiratory passages; in ancient and medieval physiology it was believed to cause sluggishness	inactivity; showing an unusual lack of energy
apathy demonstrated by an absence of emotional reactions	the property of having little or no contrast; lacking highlights or gloss
	a deficiency in flavor
	a want of animation or brilliance