voluntary has 2 distinct synonyms spread across 1 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. Its one recorded opposite is involuntary.
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.
(military) a person who freely enlists for service
of your own free will or design; done by choice; not forced or compelled
“man is a voluntary agent”
controlled by individual volition
“voluntary motions”
Synonyms, antonyms and the broader and narrower relations for voluntary come from Princeton University’s WordNet. Full sources and licences.