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
composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance
Synonyms, antonyms and the broader and narrower relations for voluntary come from Princeton University’s WordNet. Full sources and licences.
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”