declension has 9 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. 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 inflection of nouns and pronouns and adjectives in Indo-European languages
process of changing to an inferior state
a class of nouns or pronouns or adjectives in Indo-European languages having the same (or very similar) inflectional forms
“the first declension in Latin”
Synonyms, antonyms and the broader and narrower relations for declension come from Princeton University’s WordNet. Full sources and licences.