slackening vs loosening

slackening and loosening both are nouns.

Word	Noun	Adjective	Verb	Adverb
slackening	Yes	No	No	No
loosening	Yes	No	No	No

As nouns, slackening and loosening are synonyms defined as:

slackening and loosening: an occurrence of control or strength weakening

Other synonyms of slackening include relaxation.

slackening (noun)	loosening (noun)
an occurrence of control or strength weakening	the act of making something less tight
	an occurrence of control or strength weakening