relax vs slacken

relax and slacken both are verbs.

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

As verbs, relax and slacken are synonyms defined as:

Other synonyms of relax include slack, slack up.

relax (verb)	slacken (verb)
become less tense, rest, or take one's ease	become looser or slack
cause to feel relaxed	make slack as by lessening tension or firmness
make less taut	become slow or slower
become loose or looser or less tight	make less active or fast
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