announcement has 4 distinct synonyms spread across 2 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.
a formal public statement
“the government made an announcement about changes in the drug war”
a public statement containing information about an event that has happened or is going to happen
“the announcement appeared in the local newspaper”
Synonyms, antonyms and the broader and narrower relations for announcement come from Princeton University’s WordNet. Full sources and licences.