A Volunteer

1. One who enters into, or offers themself for, any service of their own free will, especially when done without pay.
2. (military) One who enters into military service voluntarily (but who, when in service, is subject to discipline and regulations like other soldiers), as opposed to a conscript.
3. (military) A voluntary member of the organized militia of a country, as distinguished from a regular or member of the standing army.
4. (law) A person who acts out of their own will without a legal obligation, such as a donor.
5. (botany, agriculture) A plant that grows spontaneously, without being cultivated on purpose; see volunteer plant in Wikipedia.
6. A native or resident of the American state of Tennessee.