fifth has 4 distinct synonyms spread across 2 of its 5 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 quantity of liquor equal to one fifth of a United States gallon
position five in a countable series of things
“he was fifth out of several hundred runners”
one part in five equal parts
Synonyms, antonyms and the broader and narrower relations for fifth come from Princeton University’s WordNet. Full sources and licences.
the musical interval between one note and another five notes away from it
coming next after the fourth and just before the sixth in position