decay has 8 distinct synonyms spread across 5 of its 8 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.
the process of gradually becoming inferior
a gradual decrease; as of stored charge or current
an inferior state resulting from the process of decaying
“the corpse was in an advanced state of decay”
the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation
lose a stored charge, magnetic flux, or current
“the particles disintegrated during the nuclear fission process”
undergo decay or decomposition
“The body started to decay and needed to be cremated”
Synonyms, antonyms and the broader and narrower relations for decay come from Princeton University’s WordNet. Full sources and licences.