volunteer

noun

a person who chooses freely to do or offer to do something
a person who chooses freely to enter naval or military service, without being compelled to do so by law
Bot. a volunteer plant
Law
1. a person who chooses freely to enter into any transaction with no promise of compensation
2. a person to whom property is transferred without valuable consideration

adjective

composed of volunteers, as an army
serving as a volunteer, usually without compensation
of a volunteer or volunteers
Bot. growing from seed that has fallen naturally to the ground, not planted by a person

transitive verb

to freely choose to offer or give without being asked or obliged

intransitive verb

to freely choose to enter or offer to enter into any service; enlist

See volunteer in American Heritage Dictionary 4