done willingly, not because you are forced 自愿的；志愿的；主动的；自告奋勇的
a voluntary agreement
自愿协议
Attendance on the course is purely voluntary.
听这门课纯粹是自愿的。
to pay voluntary contributions into a pension fund
自愿向退休基金交款
(BrE)
He took voluntary redundancy.
他选择了自愿裁汰。