turn down vs decline

turn down is not a noun while decline is a noun.

Word	Noun	Adjective	Verb	Adverb
turn down	No	No	Yes	No
decline	Yes	No	Yes	No

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

Other hypernyms of turn down include worsen.

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

Other hypernyms of turn down include refuse.

As verbs, turn down and decline are synonyms defined as:

Other synonyms of turn down include pass up, refuse, reject.

turn down (noun)	decline (noun)
	a downward slope or bend
	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

turn down (verb)	decline (verb)
make lower or quieter	grow smaller
take a downward direction	grow worse
reject with contempt	show unwillingness towards
not accept as true	inflect for number, gender, case, etc.
refuse entrance or membership	go down
	fall in value
	not accept as true