flush has 31 distinct synonyms spread across 11 of its 18 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 period of greatest prosperity or productivity
a rosy color (especially in the cheeks) taken as a sign of good health
a poker hand with all 5 cards in the same suit
a sudden rapid flow (as of water)
“he heard the flush of a toilet”
glow or cause to glow with warm color or light
“the sky flushed with rosy splendor”
having an abundant supply of money or possessions of value
“an affluent banker”
3 of the 18 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for flush.
Synonyms, antonyms and the broader and narrower relations for flush come from Princeton University’s WordNet. Full sources and licences.