piedmont has 1 distinct synonym 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. 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.
the plateau between the coastal plain and the Appalachian Mountains: parts of Virginia and North and South Carolina and Georgia and Alabama
a gentle slope leading from the base of a mountain to a region of flat land
the region of northwestern Italy; includes the Po valley
Synonyms, antonyms and the broader and narrower relations for piedmont come from Princeton University’s WordNet. Full sources and licences.