decrease vs waning

decrease and waning both are nouns.

Word	Noun	Adjective	Verb	Adverb
decrease	Yes	No	Yes	No
waning	Yes	Yes	No	No

As nouns, waning is a hyponym of decrease; that is, waning is a word with a more specific, narrower meaning than decrease:

Other hyponyms of decrease include shrinkage, shrinking, casualty, sinking, attrition, rarefaction, dwindling, dwindling away.

decrease (noun)	waning (noun)
the act of decreasing or reducing something	a gradual decrease in magnitude or extent
the amount by which something decreases
a change downward
a process of becoming smaller or shorter

decrease (adjective)	waning (adjective)
	(of the Moon) pertaining to the period during which the visible surface of the moon decreases

decrease (verb)	waning (verb)
decrease in size, extent, or range
make become smaller