impeach has 3 distinct synonyms spread across 1 of its 3 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.
challenge the honesty or veracity of
“the lawyers tried to impeach the credibility of the witnesses”
charge (a public official) with an offense or misdemeanor committed while in office
“The President was impeached”
bring an accusation against; level a charge against
“The neighbors accused the man of spousal abuse”
Synonyms, antonyms and the broader and narrower relations for impeach come from Princeton University’s WordNet. Full sources and licences.