unalterability has 1 distinct synonym spread across 1 of its 2 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 alterability.
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 quality of not being alterable
the quality of being fixed and unchangeable
“the fixedness of his gaze upset her”
Synonyms, antonyms and the broader and narrower relations for unalterability come from Princeton University’s WordNet. Full sources and licences.