barn has 1 distinct synonym spread across 1 of its 2 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.
an outlying farm building for storing grain or animal feed and housing farm animals
(physics) a unit of nuclear cross section; the effective circular area that one particle presents to another as a target for an encounter
Synonyms, antonyms and the broader and narrower relations for barn come from Princeton University’s WordNet. Full sources and licences.