a person who voluntarily offers himself or herself for a service or undertaking.
a person who performs a service willingly and without pay.
Military. a person who enters the service voluntarily rather than through conscription or draft, especially for special or temporary service rather than as a member of the regular or permanent army.
Law. a person whose actions are not founded on any legal obligation so to act. a person who intrudes into a matter that does not concern him or her, as a person who pays the debt of another where he or she is neither legally nor morally bound to do so and has no interest to protect in making the payment.