The Collaborative International Dictionary of English v.0.48:

Voluntary \Vol"un*ta*ry\, a. [L. voluntarius, fr. voluntas will,
   choice, from the root of velle to will, p. pr. volens; akin
   to E. will: cf. F. volontaire, Of. also voluntaire. See
   Will, v. t., and cf. Benevolent, Volition,
   Volunteer.]
   1. Proceeding from the will; produced in or by an act of
      choice.
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            That sin or guilt pertains exclusively to voluntary
            action is the true principle of orthodoxy. --N. W.
                                                  Taylor.
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   2. Unconstrained by the interference of another; unimpelled
      by the influence of another; not prompted or persuaded by
      another; done of his or its own accord; spontaneous;
      acting of one's self, or of itself; free.
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            Our voluntary service he requires.    --Milton.
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            She fell to lust a voluntary prey.    --Pope.
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   3. Done by design or intention; intentional; purposed;
      intended; not accidental; as, if a man kills another by
      lopping a tree, it is not voluntary manslaughter.
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   4. (Physiol.) Of or pertaining to the will; subject to, or
      regulated by, the will; as, the voluntary motions of an
      animal, such as the movements of the leg or arm (in
      distinction from involuntary motions, such as the
      movements of the heart); the voluntary muscle fibers,
      which are the agents in voluntary motion.
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   5. Endowed with the power of willing; as, man is a voluntary
      agent.
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            God did not work as a necessary, but a voluntary,
            agent, intending beforehand, and decreeing with
            himself, that which did outwardly proceed from him.
                                                  --Hooker.
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   6. (Law) Free; without compulsion; according to the will,
      consent, or agreement, of a party; without consideration;
      gratuitous; without valuable consideration.
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   7. (Eccl.) Of or pertaining to voluntaryism; as, a voluntary
      church, in distinction from an established or state
      church.
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   Voluntary affidavit or Voluntary oath (Law), an affidavit
      or oath made in an extrajudicial matter.

   Voluntary conveyance (Law), a conveyance without valuable
      consideration.

   Voluntary escape (Law), the escape of a prisoner by the
      express consent of the sheriff.

   Voluntary jurisdiction. (Eng. Eccl. Law) See Contentious
      jurisdiction, under Contentious.

   Voluntary waste. (Law) See Waste, n., 4.
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   Syn: See Spontaneous.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

VOLUNTARY ESCAPE. The giving to a prisoner voluntarily, any liberty not
authorized by law. 5 Mass. 310; 2 Chipm. 11; 3 Harr. & John. 559; 2 Harr. &
Gill. 106; 2 Bouv. Inst. n. 2332.