federal has 5 distinct synonyms spread across 3 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. Its one recorded opposite is unitary.
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.
being of or having to do with the northern United States and those loyal to the Union during the American Civil War
“Union soldiers”
characterized by or constituting a form of government in which power is divided between one central and several regional authorities
“a federal system like that of the United States”
a member of the Union Army during the American Civil War
2 of the 6 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 federal.
Synonyms, antonyms and the broader and narrower relations for federal come from Princeton University’s WordNet. Full sources and licences.