expert witness has no recorded synonym. 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 lay witness.
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 witness who has knowledge not normally possessed by the average person concerning the topic that he is to testify about
Synonyms, antonyms and the broader and narrower relations for expert witness come from Princeton University’s WordNet. Full sources and licences.