slacken vs loosen

slacken and loosen both are verbs.

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

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

Other hypernyms of slacken include loose.

slacken (verb)	loosen (verb)
become looser or slack	become loose or looser or less tight
make slack as by lessening tension or firmness	make loose or looser
become slow or slower	make less dense
make less active or fast	disentangle and raise the fibers of
	cause to become loose
	become less severe or strict
	make less severe or strict