Definitions from Wiktionary ( )

▸ noun:  One who enters into, or offers themselves for, any service of their own free will, especially when done without pay.
▸ noun:  (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.
▸ noun:  (military) A voluntary member of the organized militia of a country, as distinguished from a regular or member of the standing army.
▸ noun:  (law) A person who acts out of their own will without a legal obligation, such as a donor.
▸ noun:  (botany, agriculture) A plant that grows spontaneously, without being cultivated on purpose; see volunteer plant in Wikipedia.
▸ noun:  A native or resident of the American state of Tennessee.
▸ verb:  (intransitive) To enlist oneself as a volunteer.
▸ verb:  (ambitransitive) To do or offer to do something voluntarily.
▸ verb:  (transitive) To offer, usually unprompted.
▸ verb:  (transitive, informal) To offer the services of (someone else) to do something.
▸ verb:  (intransitive, botany) To grow without human sowing or intentional cultivation.

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