ballast has 3 distinct synonyms spread across 2 of its 6 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.
any heavy material used to stabilize a ship or airship
coarse gravel laid to form a bed for streets and railroads
an attribute that tends to give stability in character and morals; something that steadies the mind or feelings
a resistor inserted into a circuit to compensate for changes (as those arising from temperature fluctuations)
an electrical device for starting and regulating fluorescent and discharge lamps
Synonyms, antonyms and the broader and narrower relations for ballast come from Princeton University’s WordNet. Full sources and licences.