turn down vs decrease

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

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

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

Other hypernyms of turn down include lessen, minify.

turn down (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

turn down (verb)	decrease (verb)
make lower or quieter	decrease in size, extent, or range
take a downward direction	make become smaller
reject with contempt
not accept as true
refuse entrance or membership