multilaterally has no recorded synonym. 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 unilaterally.
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.
in a multilateral manner;so as to affect many parties or governments
“they worked out an agreement unilaterally”
Synonyms, antonyms and the broader and narrower relations for multilaterally come from Princeton University’s WordNet. Full sources and licences.