tail fin has 6 distinct synonyms spread across 3 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.
a stabilizer that is part of the vertical tail structure of an airplane
one of a pair of decorations projecting above the rear fenders of an automobile
the tail of fishes and some other aquatic vertebrates
Synonyms, antonyms and the broader and narrower relations for tail fin come from Princeton University’s WordNet. Full sources and licences.