loosening has 3 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is tightening.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an occurrence of control or strength weakening
“the relaxation of requirements”
the act of making something less tight
Synonyms, antonyms and the broader and narrower relations for loosening come from Princeton University’s WordNet. Full sources and licences.