Synonyms for Voluntary (same or very similar meaning)
WordNet sense 1 ((military) a person who freely enlists for service):
military volunteerWordNet sense 2 (without payment):
volunteerWordNet sense 3 (not brought about by coercion or force):
willingWordNet sense 4 (done by design):
intentional, willfulFrom the ODE community, based on WordNetadd/edit