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stiffen has 3 distinct synonyms spread across 1 of its 3 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 loosen.
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.
make stiff or stiffer
“Stiffen the cream by adding gelatine”
Synonyms, antonyms and the broader and narrower relations for stiffen come from Princeton University’s WordNet. Full sources and licences.
severely restrict in scope or extent
“tighten the rules”