witness has 11 distinct synonyms spread across 4 of its 7 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.
(law) a person who attests to the genuineness of a document or signature by adding their own signature
(law) a person who testifies under oath in a court of law
be a witness to
“She witnessed the accident and had to testify in court”
perceive or be contemporaneous with
“We found Republicans winning the offices”
Synonyms, antonyms and the broader and narrower relations for witness come from Princeton University’s WordNet. Full sources and licences.