welter has 7 distinct synonyms spread across 2 of its 4 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.
toss, roll, or rise and fall in an uncontrolled way
“The shipwrecked survivors weltered in the sea for hours”
a confused multitude of things
1 of the 4 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for welter.
Synonyms, antonyms and the broader and narrower relations for welter come from Princeton University’s WordNet. Full sources and licences.