flatness vs phlegm

flatness and phlegm both are nouns.

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

As nouns, flatness and phlegm are synonyms defined as:

Other synonyms of flatness include languor, lethargy, sluggishness.

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