volunteer

noun | \ ˌvä-lən-ˈtir \ | vol·un·teer

1. : one that voluntarily undertakes something; especially : one who without request, obligation, or an interest pays the debt of another and is denied reimbursement from subrogation : one who without request, obligation, or an interest pays the debt of another and is denied reimbursement from subrogation

2. : one who receives property without giving valuable consideration