Learn That Word

Synonyms for Voluntary (same or very similar meaning)

WordNet sense 1 ((military) a person who freely enlists for service):
military volunteer

WordNet sense 2 (without payment):

WordNet sense 3 (not brought about by coercion or force):

WordNet sense 4 (done by design):
intentional, willful

From the ODE community, based on WordNetadd/edit