decline has 17 distinct synonyms spread across 9 of its 11 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 improvement.
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.
inflect for number, gender, case, etc.
“in many languages, speakers decline nouns, pronouns, and adjectives”
a condition inferior to an earlier condition; a gradual falling off from a better state
a gradual decrease; as of stored charge or current
a downward slope or bend
Synonyms, antonyms and the broader and narrower relations for decline come from Princeton University’s WordNet. Full sources and licences.