capital has 10 distinct synonyms spread across 6 of its 11 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.
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one of the large alphabetic characters used as the first letter in writing or printing proper names and sometimes for emphasis
“printers once kept the type for capitals and for small letters in separate cases; capitals were kept in the upper half of the type case and so became known as upper-case letters”
a center that is associated more than any other with some activity or product
“the crime capital of Italy”
a book written by Karl Marx (1867) describing his economic theories
2 of the 11 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for capital.
Synonyms, antonyms and the broader and narrower relations for capital come from Princeton University’s WordNet. Full sources and licences.