What is another word for "voluntary"?

Roget's Thesaurus (1911) lists these synonyms for "voluntary": abecedary, academic, academy, accord, act, acting, action, against, agency, alma, although, ambo, amphitheater, and, antagonism, antagonistic, antagonize, art, auditorium, autocratic, bear, beat, bid, block, boarding, book, box, bring, casual, cathedra, causation, chair, check, choice, clash, class, college, collegiate, collision, come.