unconcern has 2 distinct synonyms 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 concern.
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 trait of remaining calm and seeming not to care; a casual lack of concern
a feeling of lack of concern
Synonyms, antonyms and the broader and narrower relations for unconcern come from Princeton University’s WordNet. Full sources and licences.