indifferent has 8 distinct synonyms spread across 6 of its 10 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.
(often followed by `to') lacking importance; not mattering one way or the other
“whether you choose to do it or not is a matter that is quite immaterial (or indifferent)”
4 of the 10 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for indifferent.
Synonyms, antonyms and the broader and narrower relations for indifferent come from Princeton University’s WordNet. Full sources and licences.