The Oxford Thesaurus An A-Z Dictionary of Synonyms - voluntary adj.

1) free, elective, willing, spontaneous, unsolicited, unbidden, unasked, gratuitous, contributed: All work done on behalf of the cause is voluntary. 2) discretionary or discretional, unconstrained, intentional, wilful, deliberate, intended, premeditated, planned, volitional, optional: His confession was entirely voluntary, and he was not coerced in any way.