decline vs decrease

decline and decrease both are nouns.

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

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

Other hypernyms of decline include decrement.

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

Other hypernyms of decline include decrement.

As verbs, decrease is a hypernym of decline; that is, decrease is a word with a broader meaning than decline:

Other hypernyms of decline include diminish, fall, lessen.

decline (noun)	decrease (noun)
a downward slope or bend	the act of decreasing or reducing something
a gradual decrease; as of stored charge or current	the amount by which something decreases
change toward something smaller or lower	a change downward
a condition inferior to an earlier condition; a gradual falling off from a better state; decline	a process of becoming smaller or shorter

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