decline vs wane

decline and wane both are nouns.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
wane	Yes	No	Yes	No

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

As verbs, decline and wane are synonyms defined as:

Other synonyms of decline include go down.

decline (noun)	wane (noun)
a downward slope or bend	a gradual decline (in size or strength or power or number)
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	wane (verb)
grow smaller	become smaller
grow worse	decrease in phase
show unwillingness towards	grow smaller
inflect for number, gender, case, etc.
go down
fall in value
not accept as true