affirmation has 4 distinct synonyms spread across 2 of its 4 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. Its one recorded opposite is reversal.
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 statement asserting the existence or the truth of something
the act of affirming or asserting or stating something
(religion) a solemn declaration that serves the same purpose as an oath (if an oath is objectionable to the person on religious or ethical grounds)
Synonyms, antonyms and the broader and narrower relations for affirmation come from Princeton University’s WordNet. Full sources and licences.