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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Eccl.) One who advocates voluntaryism.
      [1913 Webster]