slacken vs decrease

slacken is not a noun while decrease is a noun.

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

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

Other hypernyms of slacken include lessen, minify.

slacken (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

slacken (verb)	decrease (verb)
become looser or slack	decrease in size, extent, or range
make slack as by lessening tension or firmness	make become smaller
become slow or slower
make less active or fast