amphibian has 3 distinct synonyms spread across 3 of its 4 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 flat-bottomed motor vehicle that can travel on land or water
cold-blooded vertebrate typically living on land but breeding in water; aquatic larvae undergo metamorphosis into adult form
relating to or characteristic of animals of the class Amphibia
Synonyms, antonyms and the broader and narrower relations for amphibian come from Princeton University’s WordNet. Full sources and licences.