The Collaborative International Dictionary of English v.0.48:

Volunteer \Vol`un*teer"\, n. [F. volontaire. See Voluntary,
   a.]
   1. One who enters into, or offers for, any service of his own
      free will.
      [1913 Webster]

   2. (Mil.) One who enters into service voluntarily, but who,
      when in service, is subject to discipline and regulations
      like other soldiers; -- opposed to conscript;
      specifically, a voluntary member of the organized militia
      of a country as distinguished from the standing army.
      [1913 Webster]

   3. (Law) A grantee in a voluntary conveyance; one to whom a
      conveyance is made without valuable consideration; a
      party, other than a wife or child of the grantor, to whom,
      or for whose benefit, a voluntary conveyance is made.
      --Burrill.
      [1913 Webster]

   4. A plant, especially a flowering plant in a garden, that
      appears spontaneously without having been intentionally
      planted.
      [RDH]

The Collaborative International Dictionary of English v.0.48:

Volunteer \Vol`un*teer"\, a.
   Of or pertaining to a volunteer or volunteers; consisting of
   volunteers; voluntary; as, volunteer companies; volunteer
   advice.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Volunteer \Vol`un*teer"\, v. t. [imp. & p. p. Volunteered; p.
   pr. & vb. n. Volunteering.]
   To offer or bestow voluntarily, or without solicitation or
   compulsion; as, to volunteer one's services.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Volunteer \Vol`un*teer"\, v. i.
   To enter into, or offer for, any service of one's own free
   will, without solicitation or compulsion; as, he volunteered
   in that undertaking.
   [1913 Webster]

WordNet (r) 3.0 (2006):

volunteer
    adj 1: without payment; "the soup kitchen was run primarily by
           unpaid helpers"; "a volunteer fire department" [syn:
           unpaid, volunteer(a)]
    n 1: (military) a person who freely enlists for service [syn:
         volunteer, military volunteer, voluntary] [ant:
         conscript, draftee, inductee]
    2: a person who performs voluntary work [syn: volunteer,
       unpaid worker]
    3: a native or resident of Tennessee [syn: Tennessean,
       Volunteer]
    v 1: tell voluntarily; "He volunteered the information"
    2: agree freely; "She volunteered to drive the old lady home";
       "I offered to help with the dishes but the hostess would not
       hear of it" [syn: volunteer, offer]
    3: do volunteer work

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "volunteer":
   arbitrary, autonomous, autonomousness, autonomy, come forward,
   discretional, discretionary, do independently, do voluntarily,
   elective, free, free will, gratuitous, gratuitousness,
   independence, independent, need no prodding, nonmandatory, offered,
   optional, proffered, self-acting, self-action, self-active,
   self-activity, self-determination, self-determined,
   self-determining, spontaneity, spontaneous, spontaneousness,
   step forward, unasked, unbesought, unbidden, uncalled-for,
   uncoerced, uncompelled, unforced, unforcedness, uninfluenced,
   uninvited, unpressured, unprompted, unrequested, unrequired,
   unsolicited, unsought, voluntariness, voluntarism, voluntary,
   voluntaryism, volunteer militia, volunteer navy, volunteering,
   willful