rejection has no recorded synonym. 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. Its one recorded opposite is acceptance.
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 act of rejecting something
“his proposals were met with rejection”
Synonyms, antonyms and the broader and narrower relations for rejection come from Princeton University’s WordNet. Full sources and licences.
the state of being rejected
(medicine) an immunological response that refuses to accept substances or organisms that are recognized as foreign
“rejection of the transplanted liver”
the speech act of rejecting