1. A person who performs or offers to perform a service voluntarily: an information booth staffed by volunteers; hospital volunteers.
2. A person who chooses to enter a branch of the military without being drafted or forced to do so by law.
3. Law A person who works without pay or assumes an obligation to which he or she is not a party or otherwise interested.
4. Botany A cultivated plant growing from self-sown or accidentally dropped seed.