declaration has 6 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. 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.
a statement that is emphatic and explicit (spoken or written)
(law) unsworn statement that can be admitted in evidence in a legal transaction
“his declaration of innocence”
a statement of taxable goods or of dutiable properties
a formal public statement
“the government made an announcement about changes in the drug war”
a formal expression by a meeting; agreed to by a vote
Synonyms, antonyms and the broader and narrower relations for declaration come from Princeton University’s WordNet. Full sources and licences.