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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The Collaborative International Dictionary of English v.0.48:

Voluntary \Vol"un*ta*ry\, n.; pl. Voluntaries.
   1. One who engages in any affair of his own free will; a
      volunteer. [R.] --Shak.
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   2. (Mus.) A piece played by a musician, often extemporarily,
      according to his fancy; specifically, an organ solo played
      before, during, or after divine service.
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   3. (Eccl.) One who advocates voluntaryism.
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WordNet (r) 3.0 (2006):

voluntary
    adj 1: of your own free will or design; done by choice; not
           forced or compelled; "man is a voluntary agent";
           "participation was voluntary"; "voluntary manslaughter";
           "voluntary generosity in times of disaster"; "voluntary
           social workers"; "a voluntary confession" [ant:
           involuntary, nonvoluntary, unvoluntary]
    2: controlled by individual volition; "voluntary motions";
       "voluntary muscles" [ant: involuntary]
    n 1: (military) a person who freely enlists for service [syn:
         volunteer, military volunteer, voluntary] [ant:
         conscript, draftee, inductee]
    2: composition (often improvised) for a solo instrument
       (especially solo organ) and not a regular part of a religious
       service or musical performance

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "voluntary":
   Vorspiel, advised, aimed, aimed at, alternative, arbitrary,
   autonomous, avant-propos, breakthrough, calculated, chosen,
   conative, concert overture, conscious, considered, contemplated,
   curtain raiser, deliberate, deliberated, descant, designed,
   discretional, discretionary, disjunctive, dramatic overture,
   elected, elective, envisaged, envisioned, exordium, foreword, free,
   free will, front matter, frontispiece, gratuitous, independent,
   innovation, intended, intentional, introduction, knowing, leap,
   meant, meditated, nonmandatory, of design, offered,
   operatic overture, optional, overture, planned, postulate,
   preamble, preface, prefix, prefixture, preliminary, prelude,
   premeditated, premise, presupposition, proem, proffered, projected,
   prolegomena, prolegomenon, prolepsis, prologue, proposed, protasis,
   purposed, purposeful, purposive, self-acting, self-active,
   self-determined, self-determining, spontaneous, studied,
   teleological, unasked, unbesought, unbidden, uncalled-for,
   uncoerced, uncompelled, unconstrained, unforced, uninfluenced,
   uninvited, unpressured, unprompted, unrequested, unrequired,
   unsolicited, unsought, vamp, verse, volitional, volunteer, willful,
   willing, witting



Bouvier's Law Dictionary, Revised 6th Ed (1856):

VOLUNTARY. Willingly; done with one's consent; negligently. Wolff, Sec. 5.
     2. To render an act criminal or tortious it must be voluntary. If a
man, therefore, kill another without a will on his part, while engaged in
the performance of a lawful act, and having taken proper care to prevent it,
he is not guilty of any crime. And if he commit an injury to the person or
property of another, he is not liable for damages, unless the act has been
voluntary or through negligence, as when a collision takes place between two
ships without any fault in either. 2 Dobs. R. 83 3 Hagg. Adm. R. 320, 414.
     3. When the crime or injury happens in the performance of an unlawful
act, the party will be considered as having acted voluntarily.
     4. A negligent escape permitted by an officer having the custody of a
prisoner will be presumed as voluntary; under a declaration or count
charging the escape to have been voluntary, the party will, therefore, be
allowed to give a negligent escape in evidence. 1 Saund. 35, n. 1. So Will.