rein has 4 distinct synonyms spread across 4 of its 6 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.
one of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse
any means of control
“he took up the reins of government”
Synonyms, antonyms and the broader and narrower relations for rein come from Princeton University’s WordNet. Full sources and licences.