loosen vs slacken

loosen and slacken both are verbs.

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

As verbs, slacken is a hyponym of loosen; that is, slacken is a word with a more specific, narrower meaning than loosen:

Other hyponyms of loosen include relax, unbend, remit, slack, unscrew, unscrew.

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