brace has 27 distinct synonyms spread across 9 of its 13 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
a support that steadies or strengthens something else
“he wore a brace on his knee”
a set of two similar things considered as a unit
either of two punctuation marks ({ or }) used to enclose textual material
a rope on a square-rigged ship that is used to swing a yard about and secure it
elastic straps that hold trousers up (usually used in the plural)
an appliance that corrects dental irregularities
Synonyms, antonyms and the broader and narrower relations for brace come from Princeton University’s WordNet. Full sources and licences.