slacken vs relax

slacken and relax both are verbs.

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

As verbs, slacken and relax are synonyms defined as:

Other synonyms of slacken include slack, slack up.

slacken (verb)	relax (verb)
become looser or slack	become less tense, rest, or take one's ease
make slack as by lessening tension or firmness	cause to feel relaxed
become slow or slower	make less taut
make less active or fast	become loose or looser or less tight
	make less active or fast
	become less severe or strict
	make less severe or strict
	become less tense, less formal, or less restrained, and assume a friendlier manner