strengthening 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 weakening.
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.
becoming stronger
the act of increasing the strength of something
Synonyms, antonyms and the broader and narrower relations for strengthening come from Princeton University’s WordNet. Full sources and licences.