noun
a person who freely offers to take part in an enterprise or undertake a task
adjective
done, given, or acting of one's own free will
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Participation in the study is completely voluntary and will not affect your grade.
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You don't have to come to the party if you don't want to, it's totally voluntary.
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Joining the club is totally voluntary, no pressure.
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Her decision to help out was voluntary, like a breath of fresh air in a stuffy room.