waning vs decrease

waning and decrease both are nouns.

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

As nouns, decrease is a hypernym of waning; that is, decrease is a word with a broader meaning than waning:

Other hypernyms of waning include drop-off, lessening.

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

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

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